Each account starts with enough pending upvote balance to get their first upvote immediately. So really you're looking for how long does it take to replenish the pending upvote balance between votes. By my current estimate, for 10 units it's about 10 days.

HIVE price is in a decline right now, unfortunately, so the recharge times are creeping up.

Edit: Meant to respond on @svanbo's comment, but grabbed the wrong depth after clicking up through the thread for context.