My question is very simple and straightforward. How much time should I wait before I restart a closed proposal? Its been over 2months IIRC that particular proposal is closed. Can I restart now?
I don't see why you can't restart it whenever you like. You might consider, though, taking some time to think about why the proposal was closed or failed, and what your new proposal will do to address those concerns. If you have a solid understanding of this and are able to articulate it well to potential committers, the proposal stands a better chance of succeeding.
So I'd say go ahead and try again whenever you want, but if you want to succeed make sure you've assessed the weaknesses of the previous proposal first.