There are a ton of people who are ON for years however that's not a cycle just a bad decision that will likely lead to permanent consequences.
As many have said the longest practical cycle is 16 wks but if you are training for a contest and started too late to take a break and restart then yeah that can be a reason to overextend, That is to say (IMO) if you are training for a contest you are better off starting a few weeks earlier and splitting it into two cycles with a bridge between as the gains will be far superior. But that's just my opinion others may feel differently.
Anyhow as far as the original post, I think if you hope to cycle and recover, its best to use short to medium esters and short to moderate cycles of 8-12 weeks with PCT and time off. Rather than extend the cycles to 16-24 wks and do less cycles. In the long run with this approach you will see more gains and have less chance of damage.