Stranded in Space will be broken into multiple parts.
The first part will be 21 days. The next update will be Days 17-18, then 19-20, then 21, the final day. Then comes the next iteration of Stranded in Space, which will have a ton of improvements, changes, and better features. The total amount of days for the next iteration is yet undetermined, mostly my focus is on creating a worthwhile, satisfying amount of content per day.
In relation to your preference of waiting for a good chunk of days to play through—I get that, but each day is pretty long, especially beyond Day 12. Day 16, from a script length, is the longest day released so far (at 12k script lines). This is assuming you play through the different choices, like Elsie's bisexuality/Gaia scenes, as an example.
To answer your last question, I would love it if this next update (Days 17-18) could be done in 75-85 ish days. I can't guarantee anything but I'm trying!