12 weeks is the max id go with. to be honest my gains subside completely after week 10 anyway. People in the Uk tend to do shorter cycles. many doing 8 or 10 weeks. 12 is usually max unless your competing.
You're a UK boy too? I'm starting to see more and more guys on here from the UK.
I'd say 8 weeks is a bit on the short side but everyone is different.