It isn't easy sometimes figuring out how to maximize AB's with low durability players. With catchers it is probably the easiest to figure out. Players with durability below 75, I don't think about AB's, more in the terms of games started or played and innings. I know I can get 110 starts from a catcher with a 65 durability if I rest him at intervals and put him in the line up in the 8 spot. That is one of the reasons I like PC catchers so much because they are not an integral part of my line up but I do like them to be interchangeable.
However there are always those great players that have a low durability in the mid to low 70's that are considered an integral part of the line up. It is possible to start them for 130 games and get over 500 AB's out of them but it can take some effort on your part without taxing them. Maximizing them can be difficult to say the least. Having the flexibility at putting them in the 5th thru 7th spot in the line up is always helpful. Being able to rest them against a certain type of pitcher is very helpful, like lefties. Most generally I would say they need to be rested (completely out of the line up) at least 3 and maybe 4 games in every 20. They don't need to be consecutive games either. That does not include being replaced late in games which also helps. Another thing to consider is the stadium you are playing in or a series in the near future, always look ahead. If your low durability guy is a power hitter, rest him in the heavy pitchers parks.
One of the things I try to do is have a good serviceable bench hitter that can play the positions of my troublesome durability guys. This year I don't exactly have that luxury but I can juggle the lineup enough to accommodate my best bench hitter and get the most out of him also.
Injuries while not a good thing can also help out, that player is resting at least whether he is a low durability guy or not..lol..but it allows you to call up a player from AAA that can help with the situation. I have several at AAA with options that I can call up and wouldn't be afraid to use heavily for a short time.
There is no hard fast rule that I can actually quote or come up with as most generally I play it by ear. Of course I have had a lot of practice and know how to do it and it is never simple or the same. What I can tell you, if you leave him in the game from the start of the season, he will hit his fatigue level by about game 50 where you will be forced to rest him. After that it can be real tough to keep him rested. One thing I try to do and keep in mind is that the position players should be around or just over 300 AB's at All-Star break. If I have done that then I know I have done it right plus a three day rest period.