I think the longest I've went in between trims was about 10 weeks. The fronts were flaring pretty bad by then. I usually try & have them done ever 4 to 6 weeks. If I'm riding a lot I can wait about 8 weeks. My buckskin's feet don't grow that fast, but she doesn't wear them evenly (I think its her conformation). I have to keep her on a 4 week schedule to keep the flairing under control.
I would say that waiting until May is too long to go into between trims, but my horse aren't in a large pasture.