I couldn't find this answer anywhere.

Why would it be different from any other position player?

In HBD they fatigue significantly slower than position players. I mean, they're only doing 50% of what other players do. Logic would say they fatigue more slowly.

They fatigue at the same rate as any other position in SIM baseball, but most of us think players can hit pretty close to full ability down to 93%. With the 10% buffer, that gives you a bonus of 17%. So depending on your ballpark, the league you're in, and the place the DH will hit in the lineup, you'll want want anywhere from 500-650 PAs, a little less if you plan on having them pulled for a scrub now and then.

Depends on his position in the batting order, same as anyone else. if he fatigues just rest him for a game or 2. I use my bench so I'm more concerned about total team PA. In a DH league you need 450-500 more PA than in a league where pitchers bat.