It takes at least two weeks to fix cracks. This is because it takes at least that long for the guitar to acllimate to the proper humidity. My shop is kept at around 44% humidity and 70 degrees year round. Once acclimated the cracks almost always close up by themselves allowing me to glue them with hyde glue. This avoids the need for the use of a piece of wood behind the crack to reinforce it. The wood can change the sound of the guitar and I try to avoid it if at all possible.

I will not fix a crack unless the owner agrees to use a humidifier in the guitar to keep it from cracking further