@Terror_of_the_Night, Infinitum currently has no special rules in that regard. To be clear:
Infinitum has many features that are planned to support its vision but that will be rolled out as it continues to run. This includes Empire Surrender, Official Alliances, and the Planet-Size Protection Threshold.
It will allow what you describe, eventually, but until non-delete Surrender is ready it won’t treat inactives any differently than what is currently being done everywhere else.
That is, if they are marked inactive (which would be strange in a solo galaxy) it will be within 48 hours, otherwise it will be 7 days.
Again though, this is not intended to be the long term plan for infinitum, and the Surrender feature will factor in Infinitum’s allowance for players to take an indefinite hiatus accordingly.