Most people will say 8 to 10. The higher the light, the harder it is to keep a long photoperiod, at least for myself. I know some people like to use extremely high light but only do 5-6 hours per day. That's not enjoyable to me.
In my experience, in high light, the shorter the better, within reason. I would say 6-8. However, that isn't very enjoyable to me.
If you want a long viewing period, but want to maintain high light, there is two ways to deal with it. First is noon burst. You basically have a low/medium light tank for most of the day. Then you have more bulbs come on for anywhere from 2-4 hours a day, longer if you can get that to work. I even had 3 stages of lights for awhile which sounds cooler than it was so I end up going to 2 stages to get rid of a timer and have more outlets.
The other way of extending your viewing time is running a split photoperiod. Kind of opposite of the noon day, you have your lights go out for 3-4 hours in the middle of the day. It really doesn't extend your viewing time, it just is off when you aren't home, or are busy. I like this way as even though I work at home most of the time, I really only want to see my tank in the morning, and in the late afternoon/evening till bedtime.
Anyway, both of those are more for getting the most viewable time of your tank more than anything else. Noon burst is more popular than split photoperiod. If you just want to reduce algae and don't care, I would shorten your photoperiod significantly, like 6 hours at first, then work your way back up to where you are not getting algae. I usually cut mine off when my stems start to close up as that is when they are "done for the day", without getting to deep into explanation.