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I haven't encountered the issue of an unfinished game in a while, but I remember there was a situation at the Adelaide tournaments regarding finishing a game, and the essence of it was as follows:
If a game is postponed, the priority remains the exact time it started on any of the subsequent days... and the most important day is exactly one week after the agreement.
In general, you tell your opponent - I'm ready tomorrow, the day after tomorrow, etc., at the same time. If one of you can't, then the deadline is in a week at the same time - whoever can't make it loses!
P.S. I agree that sometimes a week is not enough, but 10 days seems awkward, and two weeks is too long.