when u recrystalise, the gunk grows on the outside of the crystal. if u evaporate it nice and slow, you will have big crystals, with crap on the outside... wash it and the crap will disolve into the acetone, and not so much of the meth will have had the chance to disolve.
water and ethanol can hold quite a bit of meth, so not much is needed. if left in sunlight or just out in the open, it will take maybe a couple of hours to 12+hrs... its mainly the last bit that takes the longest... when the crystals are there, but are still slightly wet.
you can always heat it to speed it up, but that negates the whole process and grows small crystals.
Chemelion