I potted all of my comfrey root cutting in some small pots and tended them into something suitable for transplanting, which took a week for the larger more vigorous ones, 2 weeks for the smaller. By that point the roots were very well developed and easy to transplant. Even with the recent monsoon like weather followed by a short heat wave they've all survived very well, including the ones still in waiting for a new home. It's one of the easiest plants I've dealt with to propagate, but by the same turn is also notoriously hard to get rid of.
Though it seems to grow in many soil conditions, given a supply of nitrogen it can be a heavy feeder, so be prepared to deliver some animal manure or diluted urine throughout the year. A bit pesky at times, the other benefits make it worthwhile: excellent mulch, compost activator (it has a low C:N ration reported, variously, as 9:1, 10:1, or 14:1), liquid fertilizer, and as an additive to a home-made potting mix.