I normally keep my interest up with the team I start my save with, but then if I leave, for whatever reason, and it takes me ages to find a job, I get bored. Even if I can get a job straight away, some 'novelty' has worn off. For example, I won the league with Portsmouth, but then got half way through the next season and was sitting in 10th and the opportunity to go to Man Utd came up. I went, and won the UEFA Cup (as it was then known) in my first season and guided Man Utd to 3rd (they were 7th when I took over). But after that I lost interest- the dynasty I wanted to create at Portsmouth was over.
Normally, I just end up in international management! For example over the years I have managed South Africa, China, England, Canada and Paraguay! I actually quite like internatinal management on FM.....