We haven't used Mercury lights for growing only MH and CFL but as far as the flowering stage goes it can take anywhere from a couple of days to 2 weeks to show sex. Just when you think nothing is gonna happen either pistals or balls appear. You have 1 more week left before you even need to start worrying, keep it on 12/12 and just be patient. We go a little crazy waiting for them to bud to but this current grow we used clones so we new they would eventually bud lol
Good luck, we hope they are females!!!