More often after spending extended periods on the tops or hoods. If I've been spending a good portion of the ride in the drops, or have had frequent opportunity to get out of the saddle to climb or stop, it may not ever be necessary. Generally speaking, it's only after about three or four hours that I ever feel the need to get out of the saddle just for relief, but once the need starts, it can be frequent - maybe every 10 minutes.