How often to feed new asparagus plants?

How often will depend on what you use to feed them. Gus is a heavy feeder.

If you use only compost then 1-2" layered on top 3-4x a year. You can't really OVER-feed them with compost.

I use manured composted on my beds and feed them liberally in early spring before spears sprout, again once harvest in done, again mid-summer, and again in the fall as a mulch for the bed after the deaf ferns are removed.

But keep in mind that compost alone won't feed them unless you have a very active soil food web established in the bed. And even then it takes time for it to work. So a handful of watered in compost doesn't do anything for them now.

If you use a good well balanced granular fertilizer you can probably get by with 2x a year. Or you can use one of the many good organic liquid fertilizers once a month when you water the bed.

Dave