Technically, most lithium 18650 cells can handle between a 0.25C to 0.5C charge rate, so if you are charging through the discharge port, then for a 20Ah battery, you could charge at 10A. However, even though you have seen 10A of regen via the CA, you won't see that continuously, even on a steep descent. Some of this depends on whether you are primarily using regen for recovery or for braking? For recovery, you have to set the regen current lower, since you need to bike to be moving in order to recover energy and too high a setting just brakes the bike to a speed where you get no recovery. It sort of all works out on it's own, whether you set the regen current higher or lower, when it comes to your battery, unless you start the long descent when your battery is fully charged, so one simple rule to remember.
The bigger concern on long descents is not to start the long descent right after a long ascent. The motor will create heat during regen almost as much it will on the ascent of the same hill. So, if your motor hits 100C on the way up, and you immediately start your descent, you can cook your motor before you get to the bottom, so a second rule to remember.