When is the correct time to move the cuttings to a larger pot. 50 percent of them have nice roots and small leaves. How much longer do I wait?

I pot them up when the roots are at least 1.5".  If the roots are crowded no need to wait.  If some have great roots but some don't I pot up only the ones with roots.  If the others have been in rooting media more than 5 weeks I'll float them out and check them over.  If there's no mold or other obvious reason why they haven't rooted I'll put them in to fresh media making sure there's lots of air getting to the cutting.  Photos will always help us give you the best advice as will giving us a general idea of your location and growing zone.

Maybe I'm clumsy, but I find that if roots are young, they can be very fragile.  It's really easy to snap them during the process of transplanting them.  And if the planting medium is loose (e.g., lots of perlite), the medium can break apart taking roots with it.  You can end up with a stick in one hand and roots in the other.

So I like to wait until the roots have aged and toughened just a bit, branching enough to form at least a minimal root ball.

I've heard accounts of rooted cuttings having a tendency to rot on the first pot up....especially when started in only perlite. I'm wondering if anyone has experienced this an has tips on preventing rot at this stage?

I've recently, for the first time, started cuttings in only perlite. They're rooting well but I'm concerned the adjustment on potting up might be different than if I had rooted them in a more standard mix...