It depends on the degree you are seeking, the program, and the school, honestly; what I did was compile a list, from dates provided, of a general time frame when I could expect to hear back from each school. Also take into consideration the program deadline for applications. Texas A&M had their deadline Dec 1, and I heard from them in mid-Jan; University of Oklahoma's was Jan 1 and I heard from them yesterday.
That being said, a general indication of when you should hear is around early-March (for both PhD and Master's), particularly for schools figuring out funding, recruitment weekends, etc. If you've applied to a school and it gets near April, that is generally around the time that I would suggest contacting the admissions office (or your POI) to check the status of your application.
Again, this is based on 2 rounds of grad school applications (one for my MA, and currently for my PhD). Other users may have more concrete advice.