Ramoma has it just right. When the Z is posted that is the day to vote for your request. Your request is much more likely to come up that way.
Marcelle
The way I've seen it is after every 24 hrs the voting resets. You have to request votes each day as the freebie changes. The best time, I have found, to request votes is when the current freebie font set has the "Z" up. Sometimes you're in luck and sometimes not. Keep in mind, you can also purchase the individual letters/designs but the fee to join the site is very reasonable and then you'll be able to download everything and not miss a thing! Good luck!
I believe they get reset once a design is chosen as a freebie in a category.