That depends on the person and their goals. You can do cardio every day of the week if you really want to, but you should give your body a day or two to rest. Also, you should not do strength training with the same muscle groups on back to back days.
All that being said, be sure to give your body adequate food fuel and avoid overtraining by taking 1 or 2 days off per week.
As Wendy said, it depends on the goals. As a general rule of thumb I recommend the following to most clients:
3-5 days of cardio
2-3 days of weight training
1-2 days of stretching
Stretching can be incorporated into each and every workout, and does not have to be designated to just 1 or 2 days of the week. Remember the workouts can be combined to have elements of all three types of workouts. As always, work smarter, not harder. There is a fine line between working out in a safe/healthy manner and over training.
The answer is completely dependent on the individuals personal goals. For general health, you need 30 minutes of activity most days of the week. For weight loss you need to be doing 60 + minutes of activity everyday. Do a lighter workout the day after a hard one… meaning if you run and do weights on Monday, just go for a walk on Tuesday.
I encourage my clients to stretch during and after a workout so that they do not forget to do it! Stretching can and should be done daily.
Most important, listen to your body and know the difference between being sore from a workout and pain. Being sore is good… being in pain is not.
As a very general rule o’ thumb…
I give my beginning clients the target of 5 hours of accumulated “intentional exercise” per week (as opposed to ‘incidental activity’.)
If they do more, the intensity usually goes down (depends on personal goals, obviously.)
If they can’t make it to 5, the intensity typically goes up.
Whether they “serve time” an hour or two at a pop or break it into several ‘doses’ of 10 minutes here, 15 minutes there usually doesn’t matter.