Kent Reeve, many times national champion and all-around nice guy, fabricated a sheet metal hanger to screw a simple kitchen timer to (which attached to either his scope base or a Pic rail). He used it to help track his time in rapid-fire strings (standing to sitting and standing to prone) in NRA highpower and long range matches.
In long range you may get 22 to 30 minutes to shoot unlimited sighters followed by twenty rounds for score. If you shoot at a place like Butner or Camp Perry you know the wind can do all kinds of crazy things in the space of half an hour, but in a match time is time and finite. You can wait out gusts or fish-tails, but when the man calls "Cease fire" you're done.