30mins 5 times a week is what you should be aiming for. Having said that, you don't need to do all 30mins at once if you cann't fit it in. It can be hard to find 30mins. It can be much easier to find 10mins 3times a day. Do 10mins when you first get up, 10mins in the middle of the day, and 10mins just before bed - it adds up.

Plus don't forget or underestimate the benefit of incidental excerise. Try wearing a pedometer - you will be surprised by just how few steps you actually walk each day. The average office worker walks less then 3000 steps a day. You should ideally be walking 10,000 + steps, just to remain healthy. Build your steps up gradually.
Take the stairs instead of the lift, park the car further away from whereever you are going, put energy into your housework (that counts too - just think how many kj's you can burn vacumning, cleaning windows, grocery shopping). the trick is to make yourself as active as you can just doing day to day stuff - because it means that when you loose weight you are going to be significantly more likely to keep it off.