You are ready to do your next hard set if all of these are true:
- Cardio. You are not breathing heavy anymore.
- Neural strength. You feel strong.
- Synergists. Synergistic muscles are ready, and will not be a limiting factor (e.g. forearms are not shaky when doing bicep curls, even though you are not targeting forearms).
- 5 reps+ target muscle. You can do at least a 5 reps set. It’s better to rest and do a set in a target range.