That's an almost impossible question to answer. Really, it all depends on:
- Your current strength level and conditioning
- Your boxing schedule
- Your time constraints, boxing aside
- Your recovery ability, based on age, diet and conditioning
- The frequency, intensity and duration of the conditioning workouts
Your best bet is to start including some conditioning outside of your boxing on days off. My suggestion would be to start with lighter, more "active recovery" type workouts; think LSD training at a light pace, say, 60% MRH. Increase the intensity gradually, working your way up in duration and frequency.
Strength training can be a little trickier; the DOMS carry-over into your primary sport/martial art can affect training. This is a trial-and-error approach, based on how you feel for boxing. If your DOMS and CNS fatigue is affecting your speed and strength, you'll have to cut back.