It really comes down to two things:
1) What is "fun" for you? Everyone has a line between fun and torture and it varies from person to person.
For some, it's beating up on weaker players on bullet/blitz and avoiding slow + serious games like the plague. They only want to see their rating climb up 1 single point at a time and couldn't come to terms with radical ideas like "learning from your losses" :)
Others prefer to get "free chess lessons" by getting their a##es kicked by a strong player regularly and ENJOY post-mortem reviewing the games (alone, with their friend or coach).
For most of us, it's somewhere in between.
2) A good metric is ask what your %age of wins are. A good thumbrule is that if it is above 60-65%, you're having it a little too easy and you should consider facing more stronger opposition in your games. Likewise, if your win %age gets too low (< 35%), find players closer to your level.
This is a little embarrassing but I have never won a game of turn-based chess with anyone rated higher than my current rating. Maybe I am taking advantage of lower rated players? I have left my new game request rating range at the 300 plus or minus. Maybe I should raise it to 300 plus and 200 minus? It seems that my rating has simply gotten higher than my actual skill.
Any ideas?