So... Phase 4 and beyond, speculation ...

[ArkhamNative]

(Edit: What Mjollnir said.)
Every time Yoshi-P mentions Early Access he has estimated 3-5 days, starting with a video he made the week before they announced everything. If there was mention of 7 days on any SE site, SE seems to have removed it. It may turn out to be more than 5 days, but I wouldn't expect it to be.

Also, whatever length of downtime SE is scheduling between phase 3 and phase 4, it's probably because they need it. Also, according to the roadmap:

* Dates and content are subject to change depending on development progress and user feedback.

Edit:
As for the OP's schedule speculation, it seems pretty good. It's no more than 1-2 days different than what I might guess.