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.
The reasonable man adapts himself to the world; the unreasonable one persists in trying to adapt the world to himself. Therefore all progress depends on the unreasonable man.
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