I mean this in all seriousness: infinite. There is always something new to learn from the OG, be it a more efficient way to solve a math problem or an alternative way to explain a verbal question. This is why it irks me a little when people say they have "done" the OG. I have been teaching GMAT for four years, and I don't think even I'm done with the OG!
We say this over and over on this site: the key to doing well on the GMAT is absolutely not how many books you have or practice problems you do. The key is working a reasonable amount of problems given your timeframe and ANALYZING those problems for lessons and takeaways. It's all about quality over quantity.
Okay, enough ranting. To answer your questions, I think that two months of study, two to three hours per day (plus a few days off), is a good amount of time to study for the GMAT assuming that you're studying the "right" way. In that time, I wouldn't expect someone studying the "right" way to get through more than the OG plus one prep guide and some practice tests (that's how much I did during my prep, and it took 15-20 hours per week for two months to do those three things). Anything more than that and you're sacrificing quality for quantity. So, I would focus on OG12 for now. The problems mostly overlap, but it could be helpful to look at OG11 a few months down the line, assuming you have that long until your test.
Happy studying!
Jim S. | GMAT Instructor | Veritas Prep
