I'm probably gonna ramble here, but...
I believe that success is not measured as a one-time deal. Everything in your life changes. Your goals, circumstances, opportunities...and the measure of success moves along with it. At one point in time, you may feel successful at achieving something. Later on, looking at things in hindsight, you may feel that your achievement wasn't enough because you're out doing bigger and better things--and you've raised the bar in what you think success is. The thing though is that you feel successful at that particular point in time. This is what I mean by "success" being measured differently at different times.
For example, you graduate college with a B.S./B.A. Then you celebrate your success. Then you think, oh that's not enough, I need to get my Master's. Then after that, PhD. Then what? Is this it? Then you look into a change of careers. It goes on and on, but you still feel successful during those little steps along the way.
I like to look at success using Maslow's hierarchy. If you remember from Psychology 101, the top of the pyramid is the "Self Actualization" stage. This, I believe, is the only stage that can not be FULLY achieved because life is so dynamic. If you feel "successful" in life now, does that mean that you've stopped living? You've ceased to be ambitious about other things in life? No. It just keeps going up and up and up to no end. It only ends in your deathbed.
And speaking of which, I think the ultimate time you hope to feel that you've succeeded in life is when in your deathbed, you can look back and say "I've done all there is to be done and I have no regrets. It is finished."
Well, that's my philosophy anyway.