I will do 15-25 reps in a single set as my absolutely bare minimum when I only have literally a minute. Two handed swings, heavy enough, and I just stop when they are about to get lousy, whether that's 13 reps or 30 but I try to pick a weight I think I'll be good for 20 reps or so with. It is just so much better than doing nothing with 60 seconds of exercise time.I actually miss high rep swings in my training. That was my bread and butter for years.
-S-