In addition to the good advice below, if you are working out that much I would suggest investing in a heart rate monitor to get a more accurate picture of how many calories you burn in a week- so that you can make sure you're eating properly!
3 x 30 mins is the minimum consistent with good health, and as Dragonchilde says, this is Spark's default. At the other end, it is a good idea to take a rest day once a week or so to let your muscles recover and get stronger, so 6 days per week is probably the maximum.
And unless you are training for an endurance event, once you get to being able to work out for 40 minutes or so, you are probably better off adding more intensity to your workouts, rather than more time.
In coming up with exercise targets, Spark relies on fairly broad averages of exercise intensity, and it is relatively easy to exceed these. Doing more than the target is a good thing, but I 'd recommend looking at how many calories you are burning in a typical week, and then update your Exercise Goals (accessible from the LH side of the Start page) with this figure. There is an option there to directly set a calorie target, rather than using Spark's days/minutes assumptions.
Fitness Minutes: (14,252)
9,666 3/10/13 6:53 P
As many as you are comfortable doing. :) Studies have shown that at least 30 minutes 3 times a week is optimal for good health. This is SP's default activity level.
What matters isn't how often you work out, but that you're eating enough. You basically work out the amount that is comfortable and sustainable, and then adjust your nutrition accordingly. The more you work out, the more you will need to eat.
If you're getting the "working out too much" message, simply update your fitness settings to reflect what you're actually doing. You may or may not get an increase in calories to support your activity.
You aren't overexercising, you simply need to update your goals.
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