Both your voting weight and your voting power influences how much your vote is worth.

When your voting power is still high, you are more likely to be able to give your penny vote to someone, but eventually, that runs out. I don't think it's strictly voting power because I've seen it where I have high voting power and still no penny votes to give or where it's lower, but I haven't voted for a few hours and I have a few pennies again. I have yet to see an explanation which fits my observations entirely. Best way to learn is to watch the post value as you vote - if the value goes up by a penny, then you're doing okay. If it doesn't, time to rest your finger for awhile!

Voting weight is more important.
As you power up SP, your vote becomes worth more money. I don't know what the magic number is though before it becomes worth more than a penny... I'll let you know when I find out!
I suspect (though haven't confirmed - simply observations thus far) that as you power up, your time for your voting power to refresh also shortens, giving you more overall voting power, even though you're still looking at penny votes for awhile. At some point, (well above penny vote level) you will be able to choose how much your vote is worth - which really is cool!

I hope this helps.