The last episode contained 3507 renders. If there are 5 more scenes left, let's assume that the total number of renders in episode 7 will reach 4000 thousand. (this is just my assumption, I don't know what hardware Tess has). In that case we will get 2 parts of 2000 renders (+ possibly animations) each.
As for "when both parts will be more or less ready", I would venture to guess that Tess' roadmap has no 1st or 2nd part (in terms of story), but a single and indivisible 7th episode. And its separation would be purely artificial. I wonder why this division is necessary at all? What's the point of it? At first, I thought it would be useful to "sweeten the waiting pill" a bit for the next six months by brightening the players' wait for the 8th update. But Tess has never done that before, and it's been fine all along, both in terms of unrelenting interest in the game and steady growth in subscribers.
This could be one of the most likely reasons for splitting episode 7 (increased development time for "super big" Ep 8), but something tells me that Tess won't want to repeat this forced lengthened development cycle again and will try to go back to the normal 6-month cycle. But that's just my guess.