I think ninety minutes per week is a good amount for a challenge. That breaks down to about thirteen minutes per day which is something even the most out of shape amongst us may be able to manage.
Yea that's what I was thinking! The government here recommend 30 minutes 3 times a week. I think it's quite an achievable task, cos even walking 15m to the shops and back would count!
It's nice to set goals at a level that people can achieve so they can feel good about accomplishing them! If they are too high, people give up before even trying because it's too overwhelming.