I used to sleep a lot a lot, but have found recently that the best time for me is 7.5 hrs.. wake up after 6 and do a WBTB for one more cycle, or wake up and take an hour and a half nap, best success rates with lucids so far. Sleeping longer than that I find that if I haven't had a lucid already I probably won't or if I want to improve the mood/ theme/clarity of my dreams it's usually unsuccessful after that point.. unless I utilize sleep deprivation and get 6 hrs several nights in a row and then get 9-12 then it's super dreamland.