I had laser treatment from age 6 weeks to 10 years (about 12/14 treatments in total - I can't remember). I didn't notice any lightening in the first few treatments, the skin does redden straight after due to the laser on the skin. It may take a couple of weeks to go back to normal- so don't worry about that. Places that have more nerve endings (e.g. palm of hand/wrist area) can talk longer to go back to normal and can be more painful.
After a few laser treatments the colour started to fade. I was told when I was 10 that anymore treatment wasn't going to affect it that much, so I haven't had anymore treatment.
I am now 18, and it has got gradually darker again. I have done research into more laser treatment but because PWS tend to be deeper than the laser can reach, the birthmark will never fully disappear. Apparently there is testing going on in the USA for a new from of treatment.
I hope this helps.