How you do your repetitions in your exercise movement is far more important than how many repetitions you do. Your exercise should be about quality not quantity. Because it should be about quality, you will focus and perform each exercise movement with purpose and diligence. It is common practice to go by the number of repetitions. If you alter that practice and go with some other factor (like time), then you will focus on quality. For example, you could try to do 35 seconds of lunges rather than 10-12 repetitions of lunges. Later, after you master your form, you can go by repetitions on lunges.
In addition, you can over extend yourself on certain exercises. From my personal experience, I have tried to max out on pull-ups. I ended up hyper-extending my shoulder joints. This impacted my overall upper body routine. I should have focused initially on mastering my pull-ups. 2-3 repetitions of good form of pull-up are more beneficial (and safe) than 10-12 awkward forced repetitions of pull-ups. By keeping my abdominal muscles tight, keeping my shoulder joint tight and secured, and keeping my form proper (maybe trying them out using a chair or bench), this would have prevented injuries that could have a lingering effect on your fitness routine. As I found out, your shoulders are a delicate and intricate joint. It requires that you have mobility movements (e.g. shoulder rotations) and static stretches (e.g. shoulder pulls or hanging from a pull-up bar) with strength exercises (e.g. upright rows and pull-ups).
This is the lesson from quality not quantity.
Another example of quality over quantity pertains to running. I love running. It is one of the best exercises to get your metabolism up and going. Now, most people would assume that the longer you run the better. They would jog, walk, or run for hours like a marathon runner. Instead, for your overall fitness, it is better (and shorter) to do interval running and sprints. This would be great for your joints and overall fitness. Both a marathon runner and sprinters are excellent athletes. But, I would rather look like a sprinter than a marathon runner. It is about quality over quantity.
One final example would be push-ups. I thought and assumed that more push-ups the better. Once you master the push-up movement, you should make more complex and compounded push-up movements. It is important to first master the push-up movement. You should keep your abdominal muscles tight. You should be able to keep your body (e.g. back) straight during the movement. Later, you can do clapping push-ups, plyrometric push-ups, T (or yoga) push-ups, etc. to challenge and give more quality.