hi Tan,
The theory works really well - I used to be a member of contours but the only problem I had which ended up causing me to cancel was they they have really strange opening hours so I could never go after work or on weekends so it really is geared to people who are able to go during business hours.
A half an hour is good but if you can managed an hour or even 45 mins you'll get better results.
Good luck!!!