I have and I found that my gains plateaued after 16 weeks, and I had to add more gear per week. Its done all the time however keep in mind if you choose to run a longer cycle , you will need double the amount of time to recover and it could lead to you losing more gains than if you would have done a shorter cycle.
the only guys on here that have done a cycle 6 months long like that are guys who are cruising.. most everyone on here cycles though. the danger as stated above is the several guys who have run longer cycles and haven't been able to recover so they've gone on TRT. it sucks and you do not want that. like Arab said gains usually plateau anyway, so no reason to keep going. it sucks cycling off but the body as a whole needs that rest. your ligaments and tendons need to catch up to your muscles.
I like to keep my tren at 6 weeks flat, my mast/primo/test at 10 weeks flat. the long esther AAS like primo or test will still be in your system 2 weeks after. so really stopping at 10 weeks is really 12 weeks of fun.