I wouldn't worry about hermaphrodites right now. Some plants, depending on age, aren't even sure if they're gonna be male or female yet... so becoming a mix of both probably isn't likely to happen at such a young age. (They need more time to become fully corrupted. )

If it's something that's truly got you worried, just leave the lights on for 24 hours until you can get a timer to regulate things. You can pick up a plain analog timer at any hardware store for just a couple of dollars, and you can even get many of the digital ones at most of the same stores for just a slightly higher pricetag. So get out there and pick one up! You absolutely MUST have it for flowering, as I'm sure you already know, and if you're not wanting to keep the lights on for 24 hours a day, then it's worth the minor investment to just get out there and get it. A couple of $$$'s will save you a whole lot of headache.