Actually, the levels can be even more of a seesaw than that. Usually it's 4 posts for intro, after that, depending specific girl or stage it can require anywhere between 4-11 posts, though once or twice i have seen a stage unlock for less than its required number. Stage 1 seems to be the more expensive one for all girls, though there are exceptions like Vanessa or Felicia that tend to stick to a 5-6 average on all levels, with Wagyu-Chan being similar but slightly more expensive at an average of 7 per stage i guess.
But yeah, taking a crapload of time just to recover eyecandy from girls already conquered long ago can be agravating, even without accounting for the headache of checking every 4-6 hours so your "post capacity" doesn't get full, what will lead to gaining no more posts and more hours of time-wasting. Oh, joy. 😩
And someone needs to tell the @$$#0!&$ programming this mess that changing the interval to 2 hours per post is not anything like a decent solution, quite the opposite in fact...😠