This depends on a number of factors, but in basic terms I think there are primarily three. Firstly, you need to think about how experienced you are as an exerciser. If you are new to it, jumping in the deep end with training six times per week is unwise, not least because you may start to feel the effects of over training, which can leave you feeling tired, lethargic and susceptible to illness. If, however, you have been working out for a number of years, training intensely five or six times per week may well be fine, but I’d still advise one of those days in the middle to be a low intensity, active recovery day, which brings me onto the second factor to think about – intensity.
The more intense the workout, the longer your body will take to recover. If you like to jog for twenty minutes per day, your body should be able to recover sufficiently in between jogs because this is a low intensity activity that primarily calls upon the aerobic energy system, which is the one that can last for hours. If, however, you push extremely hard with some metabolic conditioning or a heavy strength session, it would be unadvisable to do another session of a similar intensity the very next day, because your body has not had enough time to recover. The exception may be if you are using a specific training “split” in which, for example, you hit legs one day and chest the next. This may be acceptable as you are using different muscle groups, though it will still be fairly taxing on the central nervous system.
The third factor to think about is your availability, how often are you realistically able to train. It’s great to have the mindset that you’ll fit in a workout no matter what, but at the same time don’t beat yourself up if you can only fit in two or three 15 minute HIIT sessions one week when life just gets in the way too much – progress is still being made!
Above all, listen to your body. If you’re feeling fatigued and sore and think you may benefit more from a rest day, have a rest day. You’ll know the difference between that and if you’re just being lazy. In which case, start warming up! Remember, no one knows your body as well as you do, so trust it and let that be your guide when deciding how often to train.