I'm in NW Illinois.

Planted 3 purple maiden grass last year, and they got huge.

In early April I trimmed the dead grass down to about 6".

I don't see any new growth yet. When should I expect to?

don't be surprised if they don't come up until june, especially if they
are in a shady area. i have 4 different grasses and only 2 of them
have started to come up. it also makes a difference how much organic
matter is in the soil. sandier soils seem to give grasses an earlier
start and longer season while well composted soil will encourage more
vigorous growth for a shorter period.