The boric acid will mix fairly quickly in the water. If you've got any sort of decent circulation at all, then you could swim 30 minutes after addition. In my own pool, I'd have no trouble swimming even after 10 minutes because I've done dye and chlorine tests to see how quickly chemicals mix if one pours over a return flow in the deep end (but I've got a skimmer on the opposite side and floor drains below). With large quantities added, be sure to brush the bottom of the pool where you may find a small amount of undissolved boric acid -- just sweeping it will stir it up and dissolve it.