Germinating in jiffy blocks

Hey! I'm still rubbish at searching existing threads, soooo

I'm trying out the jiffy blocks for the first time as i've had issues with transplant shock. Just wondering - how long does it usually take for them to crack? I know there are variables but a general idea, and do we want to keep them quite wet or what?

I have them in a small propogator and the blocks seem to be getting quite dry

cheers