Somehow or other, I’ve managed to complete 22 volumes. Some were longer (the Sailormoon ones could be over 350 pages), and some were short ones (I think Whisper of the Heart was around 180). And some are just plain easy (I almost read then entirety of それでも歩は寄せてくる volume 1 in a single reading session!)
I was originally reading a little each day from multiple books, spreading each book over the week. This was great because I could read a little from a more difficult manga, then switch to an easier manga. It also meant I didn’t have a week between reading to forget what’s going on. But it was also a lot more difficult for me to manage. Having a simple “read one chapter per day” is easier for me to manage.
That one was such a rare find because it works so well for easing a reader into Japanese grammar. I haven’t been able to find anything quite like it.
Are you looking for something more advanced than, say, よつばと！ is? Or have you not read that series to judge by?