The usual long cycles are 16 weeks - this time is enough to get very good results with slow acting compounds, and yet this will allow the body to get well recovered with minimal issues. Longer cycles are possible, but the benefits of additional gains are not worth the price of a more difficult recovery.
As for the compounds, a basic testosterone + EQ cycle is the best thing you can think of when it comes to long cycles and quality gains. The side effects will be minimal, and you would only need an AI - no need for prolactin control, no liver support.... The only thing I would recommend you to do on such a cycle, is to give blood somewhere in the middle of the cycle, since EQ will increase red blood cell count, making the blood thicker. Blood giving helps thin the blood, and make it more liquid.