• Forum has been upgraded, all links, images, etc are as they were. Please see Official Announcements for more information

Pre-Proposal submission question

I would say at least a week or so. A lot of people have posted a pre prop only a few days before doing the actual proposal and with only a week or less left to vote, but that is not recommended. You have plenty of time right now.

I would also recommend keeping it to no more than three months in payout length. If it will take more than one month it is best to split up the payments over a few months. If it is a large sum and there are a lot of contractual obligations you may be asked to escrow the funds with the Core team or someone else. A contract agreed to/signed by Core may be good enough without escrow.
It depends on the feedback you receive.
If there is major dispute or discussion related to the content and offering, more time is required to resolve those issues.
I wouldn't post a proposal with unresolved outstanding issues.
With that said, I would recommend submitting your proposal no later than 10 days prior to the voting deadline.
In my opinion, judging from the content of your proposal, you have plenty of time to get your proposal into this voting period.
How long after submitting a pre-proposal should you post your actual proposal?

How long is a string?

I'd say it should be relative to the feedback that you get. If you get the desired feedback after 24 hours to create a better product/solution then that is fine. If after 3 weeks no one has responded, then you may not have enough feedback to know whether your proposal will fly or not.

I'd suggest you don't like at asking for feedback on a pre-proposal in order to just tick a box. Seek feedback to help you refine your solution/project
As djcrypto says, it depends on the feedback but you can wait within 1 or 2 weeks. The most important thing is that you must have good feedback, if you dont it means that the community is not so interested in your pre-proposal.

Good luck!