How long cardboard, or several layers of newspaper, would take to be digested in the soil you have will depend on how active the Soil Food Web is. If you are starting with a soil that has little organic matter now and therefore an very inactive Soil Food Web it could take a year for the paper to be digested. However, if the soil is a good healthy soil with a very active Soil Food Web the paper may be digested in 4 to 6 months..
Then begs the question, if what you are putting on the soil that is there is going to be more than 6 inches thick and will then block access to sunlight for any plant growing in that soil is the cardboard really necessary?

Well, I don't know what the food web is like, but I don't think there's a lot of organic material in there. There were quite a few earthworms though, which was a pleasant surprise. I thought I would need to put cardboard down to smother the grass, but perhaps I'll just turn the grass over, and mulch with lucerne, horse manure, autumn leaves and woodchips.

I have had great results from turning over the sod by the shovelful, the roughing it all up and adding cardboard, compost, and mulch. I have gobs of earthworms at work, and there is little sign of the cardboard after about three months, but that is during the summer months, with heat and humidity at work.

Even with very thick (>6") mulch, I've had weed issues when I didn't use cardboard. You might try solarizing with black plastic for a couple months if weeds are a concern?

Ahh that's great news Jadie, thanks. I will definitely use that technique in beds I can prepare for later in the year. It's not really weeds that concern me, it's just the grass. There is some type of really aggressive grass in that area that sends out these big fat long shoots that root along the ground and is difficult to remove, but to be honest there is not very much of that particular type of grass there, and if it grows up through the compost I could just hand weed it. It's mostly just normal grass.

Perhaps I'll lay down cardboard in the more weedy areas, and if it's not discintegrated enough by August I can just pull it out at planting time.

I didn't really want to use black plastic over the beds because I had somehow thought to myself that the rain we expect over winter would help break everything down, and I didn't think the black plastic would help much with 'cooking' the mulch because of the cold(ish) weather. Am I wrong about that?

Sounds like your soil is not in bad shape. I would even say you didn't need to add all that you are adding. However, since you know you are going to dig there in a few months, I would definitely choose newspaper instead of cardboard. In this way, when you're ready to dig, you can do that easily, and leave anything else remaining there to decompose with time (and with the added watering for the roses, you will then be helping it along just fine)

The presence of "quite a few" earthworms indicates you do have an active Soil Food Web, because earthworms need a fairly decent amount of OM in the soil to live on, as do the rest of the Soil Food Web.
If the unwanted plant growth, aka "weeds", are of the invasive type, some grasses, "wild morning glory" etc., they could grow up through much more then 6 inches of mulch over time, but I find that often there is a question of how deep is 6 inches. My fist, from the index finger to the pinky, is 4 inches and I often find people with "6 inches" of mulch often have less than half that. Then some people do not seem to grasp that "weeds", those unwanted plants, will seed, germinate, and grow really well in organic matter put down as mulch.
Have you checked into having a good reliable soil test for soil pH and major, P, K, Ca, Mg, , levels? Maybe these simple soil te1) Soil test for organic matter. From that soil sample put enough of the rest to make a 4 inch level in a clear 1 quart jar, with a tight fitting lid. Fill that jar with water and replace the lid, tightly. Shake the jar vigorously and then let it stand for 24 hours. Your soil will settle out according to soil particle size and weight. For example, a good loam will have about 1-3/4 inch (about 45%) of sand on the bottom. about 1 inch (about 25%) of silt next, about 1 inch (25%) of clay above that, and about 1/4 inch (about 5%) of organic matter on the top.

2) Drainage. Dig a hole 1 foot square and 1 foot deep and fill that with water. After that water drains away refill the hole with more water and time how long it takes that to drain away. Anything less than 2 hours and your soil drainsâ too quickly and needs more organic matter to slow that drainage down. Anything over 6 hours and the soil drains too slowly and needs lots of organic matter to speed it up.

3) Tilth. Take a handful of your slightly damp soil and squeeze it tightly. When the pressure is released the soil should hold together in that clump, but when poked with a finger that clump should fall apart.

4) Smell. What does your soil smell like? A pleasant, rich earthy odor? Putrid, offensive, repugnant odor? The more organic matter in your soil the more active the soil bacteria will be and the nicer your soil will smell.

5) Life. How many earthworms per shovel full were there? 5 or more indicates a pretty healthy soil. Fewer than 5, according to the Natural Resources Conservation Service, indicates a soil that is not healthy.
might be of some help.

Wow, thanks for the detailed response.

I've not has a soil analysis done here, but I will do the jar test you described. Without doing any of that though, I know that tilth and drainage here are not ideal. When balled, the soil stays together in a big old clay clump. When it rains heavily, the soil becomes sludgy clay and water pools ontop and makes swampy puddles for days - and then when it's baking hot the soil becomes so hard that water just can't penetrate down to the roots ( the water just runs off downhill).

I understand that weeds are going to love my newly enriched beds :) I'm not overly worried about weeds per se, I just down want my new beds to be slightly higher grassed areas come spring! I guess if weeds love it, I'll know I've done a good job, and then get back to the business of hand weeding! I found it's not too hard to remove the big aggressive suckers by hand whilst turning the sod (which I started today) and I don't think the normal (more wimpy) grass will come through, even without cardboard. I have a LOT of mulch, you see :)