Exante officially advise 20 minutes of exercise per day, and I know some people on here do much more. You just may need some extra calories if you start to feel too weak. Maybe 4 packs a day instead of 3, or add a small meal along with the packs. Everyone is different because everyone has a different tolerance level and fitness level.
AS ABOVE, it depends what you are used to; take it slowly, don't over do it. You can build it up gradually as you adjust to the diet. If you feel a bit weak or washed out - just slow down. I now do far more than I did at the start - You feel so much better as the weight drops off and you have more energy so its easy to increase exercise then. Its very important to take it slowly. Its not a race.