A 24 hour filter could work, but as what Poi said it could be used to spam on a daily basis.

I do feel that clarity in what's available is in need of improvement, but I feel as though there are some other alternatives that would be better suited for discovering what roleplays are open. For starters, a filter on Apply, Open, and Closed would be a suitable option, but in reality not everyone looks at the Roleplay section for an RP they wish to join, but rather the Interest Check section. The interest section is an advertising board and the topics that are at the top are the fresh and current roleplays. Need a boost, bump your interest check thread. Afterall it's there to see if people are interested. 

Sadly the Apply, Open, and Closed section assumes that the GM even plays attention to this part of the thread structure. I have seen a large majority of GMs forget this part, or have become too lazy to change it... even if its right there. If you have the players you need and the interest check isn't up at the top anymore, then there is little chance that someone will pop on by and join as the Roleplay Section moves a lot faster and is less explorer friendly than the Interest Checks.