I would think several pounds of citric acid would be about 5 pounds. Let us know how that works for you. After dissolving the citric acid, and starting the regen, you could put the softener into bypass about 10 minutes into the the brine-draw (BD) cycle, or watch the level and bypass as soon as the liquid level in the brine tank is near mininum. That should let the citric acid solution sit with the resin longer. Maybe hold that for -- I don't know-- 30 minutes? Restore the bypass to normal, and maybe follow with another regeneration right after the current one completes.
Softners are better designed to finish drawing the brine after about 15 minutes, and the rest of the brine draw minutes are slow rinse. Often a higher-flow injector gets used, and the actual brine is drawn by about 10 minutes. It also could be 20 minutes. Thus watching would be best, but 10 minutes would be a good number if you don't watch. You would still have to wait out the backwash to start timing, unless you know how to step right to the BD cycle on your unit.