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This way such return proposal can be like any other without over-complicating code itself. Also it allows other return proposals to be defined later on to fine-tune funding management.

There can be more than one. For example, if stakeholders feel the entire DAO budget is too large they can create a burn proposal which burns half and vote it to the very top. The remaining half will flow down through the others proposals, effectively cutting the total budget. Or similarly if stakeholders feel that too much is being paid out now but perhaps should be saved in the treasury for later, a return proposal for part of the budget can be voted to the very top.

There are other combinations possible.

Each 24 hour period?

Payouts are every hour. 1/2400 of the pool is paid out.

So, each day is 1/100th of the pool?

We vote each proposal just once?
How long do they persist?

You can change your votes whenever you want. If a live proposal is high enough in the rankings during the hourly payout, it gets funded, if it drops down, funding is cut off. Like witnesses.

live proposal

How long do they live?

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.