For what it's worth, if you're active and go for one star girls first, the basic schedule is:
1) Within a month, you can have a full team of natural one star girls ranked up to seven stars with over 200k power each and be able to easily clear all content except for level 50 raids and the top tier of some occasional events.
2) In about 4-5 months, you can have a full team of stronger natural two- or three-star girls ranked up to seven stars with maxed talents and seven star purple gear, at around 400k power each.
3) In about 7-9 months, you hit 260 total star rank and have caught up to the people who have been playing since launch.
That assumes that they don't make it easier to catch up in that time. They likely will, as the general trend is that games make it easier to get stuff as the game is out longer. For example, at launch, no two-star girls had training missions, only three three-star girls had an ascension mission, and those ascension missions generally cost about 36 AP per shard for the level 50 version--which players couldn't do yet, anyway.
Later on, a lot more one-, two-, and three-star girls had advanced exploration, training, or ascension missions to give their shards, but the drop rate was low. There was some randomness, but the average AP cost per shard was about 15 AP for a one-star, 20 for a two-star, or 36 for a three-star. Then they increased the shard drop rates dramatically so that those figures are now 4, 8, and 10, respectively. That allows you to star up your girls much faster than you used to be able to.