Four years - the first full term, if you will. You run on a platform of all the things you're going to do; care for everyone, keep a budget, trim the fat, uphold the common good, educate your young, honor your elders, defend what is yours, and promote the values that bind you together. You compromise on all of them here and there, not because you lose your way or falter in your commitments, but because you have to compromise sometimes in order to achieve most of those goals. At the very least, you have to prioritize a bit, and in the end you always have to cut some programs that really mattered to someone.
But if you keep every promise you possibly can (even those that were ill-advised or over-stated), and always act within everyone's best interests, hopefully you'll be asked to stick around for another term.
I don't know anything about politics, but marriage sure as hell works that way.