I believe there's on specific time when you should send proposals. You can send it anytime, just study the proposal properly before sending proposal so, your connects don't get wasted.
There are different tips that make the best time to send a proposal. If you see that a client has a bunch of invites and interviews that means that chances of seeing your proposal are less and less
In my experience, as quick as possible. Every job post will have more than 50 proposals. so if you are one of the first ones, the client will view yours first. In fact, twice I received an invitation and I repplied to it with my phone. Thoses clients were online, so they started asking me more questions and I responded, and I landed those jobs!