We usually train thinking of reps, but sometimes it's better to think of time under tension or TUT. Take the wrist roller exercise. Do it on a high step to maximize the distance. The weight won't have a chance to rest on the floor and you'll keep the tension constant – one of the key ingredients of muscle growth.
Now go for time. Start with 30 seconds (15 seconds up, 15 seconds down). Make it a goal to hit the one-minute mark. It doesn't matter how many "reps" you do, just hit 60 seconds.