That's part of the proposal, and I don't know if there is a hard limit. Personally I think this is actually a weakness in the system. There is a 10 SBD fee per proposal to reduce spam, but very long-lived proposals constitute a form of spam as well, and the fee should probably be per-month or something.

In practice I guess UIs will have to prioritize newer/active proposals in some manner. I don't know how that is going to work out.