Hi,five to six weeks is the norm.Don't change to early as growers pellets are lower protein that the chick crumb so changing to early could reduce the growth rate.Hubard's as Ross Cobbs usually go on to growers pellets at 3 to 4 weeks but they grow much quicker.
I find it's usually about the same time as the chick fully feathers up,I find they get to a point where they suddenly seem to be eating chick crumb quicker than you can put it down thats when I change.
I don't normally mess about mixing the crumb with the pellets to much,I just switch them straight over,if they wont eat the pellets just sprinkle some crumb over the top,they soon get the idea.
Graham.