There are no set "rules" when it comes to ending a brumation cycle. If you have animals that you cycle annually it generally helps to keep them on roughly the same cycle each year.
Think about the factors that may have stimulated the animal to brumate originally, such as shortened day length and temperature. When those factors change he should resume natural feeding and activity regimes.
Here in the SW our problem is that the climate often makes it difficult to get a long and cold brumation. I generally put my stuff down in late Nov and "wake" them in mid to later Feb. Think about under what seasonal conditions your snake's ancestors evolved to deal with, example; is it a bull from northern latitudes with a brumation that starts in Oct and ends in late April? or is it a southern pine whose ancestors evolved with a much shorter "winter" season?
Also factor in the snake's body weight and health at the time of brumation when considering how long to keep them down.
I know this is somewhat vague but I hope it helps.
Ginter