how much should i do?

I am totally lost when it comes to how much exercise I should be doing. I cycle on my stationary bike and sometimes set myself stupid targets like, 2-3 hours a day, then sometimes I only set myself 20 minutes. I can't seem to find a time I feel comfortable with, but feel I've had a good workout. whats the recommendation? Also, is it best to combine this exercise with a toning exercise such as pilates?

Um, 2 - 3 hours is just daft, sorry...unless you are not cutting calories and even then it doesn't make too much difference to weight loss in general as exercise alone does not burn off fat like cutting calories and exercising...

How fit are you? Do you get out of puff easily? This will help you to judge what a good work out is...if you are getting out of puff and sweating and feeling tired afterwards then you are working out hard and well, if you are not getting a sweat up or getting out of breath (not to the point of exhaustion however) then you aren't really working that well or hard...unless of course you are very unfit and have to build your fitness up slowly...

I am pretty fit (still room to improve though!) and I only do 45 minutes of cardio 3 days a week (spin class) with 5 days of weight training (anaerobic exercise) and 1 day with an hour (plyometrics, cardio, core stability work and free weights) with my personal trainer. 2 days off (Saturday and Sunday) So I do 3 hours of cardio max per week (not per day!!!)

And yes, cardio plus toning is a very good idea...although core strength training (which pilates covers I believe) is more important...to begin with...