I think 50 is too light.
This guy's low 60s and he is just a tad light still IMO.
This guy is a 56 and as a 55 was used as the RH portion of a platoon, but must get rested as he only logged 740 Innings behind the plate in 90 games started, and 392 abs.
And lastly
the guy I actually use in a quasi platoon. He is a 64 ever since I acquired him, and he ALWAYS requires days off as the RH everyday starter. In S11 he got down to 97 or so and I'd rest him as needed. In S12 I tried to keep him rested periodically and feel I left innings on the table from him. I have a defensive specialist that will come in as a def replacement and to rest the starter, so the difference in IP and GS is a bit skewed from S11 to S12.
Hope these examples give you a little better idea. GL.