My knee has been hurt for almost a month now, so I'm estimating i've skating my 4's for around 15 hours. Today I noticed that they are starting to feel a little soft and slow (with super reds). I pulled out my other setup which has STF's (with mini logo bearings) that I only skated for about 2 or 3 hours, they felt harder and faster. So for whatever it's worth, after about 6-7 sessions with Formula fours they are starting to feel worn down. I normally skate the same wheels for a couple months.