Personally, I like to wait until the ferment has calmed down and the pectic enzyme has killed the foam height. Some of my recipes have too much pectin to be contained. I aim for about 1.030 to 1.020 at the latest, you need to still put out enough CO2 to fill and purge the demijohn after racking. And if the ferment has slowed you are losing the protection of the CO2 cloud.
I like to rack with all the yeast after about 4 to 5 days while still a fair activity level into a larger vessel. I use a 6.5 gallon carboy for a 5 gallon batch. Then again to a 5 gallon after 3 to 6 weeks, depending on how the progress is. I ferment fairly cool, so some are very slow. Others are clear at 3 weeks.