I qualify my BBS directionally, and a good razor gives me directional BBS. What I mean here is that I can rub my finger in the direction I shaved and not feel anything. I use the directional part because it is part of how I determine whether or not a razor is shave ready. A razor is shave ready if it gives me directional BBS and cuts cleanly. I'd never considered the time aspect; I usually check immediately after the pass or while applying my AS balm.

Note: A really good razor will give me perpendicular BBS, meaning I can shave WTG and feel BBS when rubbing XTG.