You have to clarify your question a little. Too long for what? Too long for you? Too long for health?
I know you are going to get a bunch of people tell you that you need to take time off with the same or greater than your time on. Problem is they are going to tell you that because someone told them that. There is no scientific evidence that I know of that substantiates this...just the lore of the steroid world has passed this advice down through the years.
However all the pros have no off time...they can't afford it. It is their "job" to be on all the time...they couldn't cycle and keep gaining all the size they need to get better...so 20 months on is nothing...of course you could most likely add a zero to the 250mg you are asking about as well as throw in some GH, EQ, Winny, & Deca.
Sorry to not answer your question, but guess what, no one else will be able to either with 100% accuracy.