10-12 hours... even my cobomba closes up shop for the night at about 11
hours. Equatorial tropical countries get an average 12 hours of sunlight on
any given day and most plants and fish we love to cultivate in our tanks are
usually native to those areas. So take your pick... 10 or 12. I'm sure
others on this group can tell you all the drawbacks to leaving lights on for
longer periods but I just do it for 11 hours so the fish feel more like
they're in their natural environment.