Depends on water pressure, the "seepage rate" (don't know the real term, should be on the label, I think it's expressed in gallons per foot per hour or some unit like that), soil and weather conditions. I generally just turn them on and let them run until I have a nicely saturated zone at least a foot out from the plants -- mine is a clay-loam for the most part, water may go more down than out in a sandy soil.

If you use the search to pull up all the previous discussions on using soaker hose you'll find that a common recommendation to determine time needed is to simply bury an empty can - coke can, vegetable can, something about 6" tall.

Bury it along the line of the hose in about the middle of the hose run so the open lip top sits at soil level and the hose runs over it. Time how long it takes to fill the can and that tells you how long you need to run it to get the water down to root level.

It isn't a perfect method because it can't account for your soil type so it won't tell you how often you need to run it. That part you determine with your hand stuck deep in the ground. But it will tell you how long to run the hose.

With soaker hose keep in mind that 50' maximum run is recommended for effectiveness and that the plants at the end of the run get less than the plants at the beginning of the run.

Dave