When you marry someone, is it technically in theory supposed to be for life?

Depends upon the culture and the country. And the times. Today, its desirable, but optional in most places, unless one is Catholic, I suppose... 

a hundred years ago

In marriage the partner is called life long partner legally as well as culturally . Partner can be changed if he or she is not compatible .Since the marriage is a contract the separation has to be legally and is called divorce .A wife is entitled to maintenance which will be decided by the Judge giving the verdict .

What part of "until death do us part" did you misunderstand?  What is supposed to be and what is are two different things.  From a legal stand point the law is different depending upon what state you reside so it is impossible to give you a finite answer.

One rather expects that people marrying hopes that it will last! I doubt many marry not expecting it to be "for ever"

We fall madly in love, don’t we, thinking (feeling rather) that it will last forever and is enough for a serious relationship. However, it’s very hard to live on an emotional high for very long. Eventually we start to come down from the ecstasy, excitement, strong passions and desires, typically after around 18 months to three years (people vary of course). If couples are friends, discuss their mutual values, shared ambitions, interests, etc., in some depth (obviously there will be some differences, which help make relationships interesting), and make plans, work on their personal development, etc., this can develop into a long and wonderful relationship. If one feels insecure or low in self respect, it can make for a difficult partnership. It's easy to behave at our best when in love, but marriage, for example, requires a lot of self discipline, sacrifice, compromise and flexibility. If a strong friendship is not in place, the relationship will probably peter out eventually - or worse. Quite often we fall in love because we are lonely and allow ourselves to be won over by anyone who takes an interest in us. Thus we give away control to somebody else if we are not careful. This is another reason for taking things very slowly, and really getting to know someone before committing ourselves or getting too emotionally or sexually involved. Sex can be emotionally bonding, which is disastrous if the other things are not there: strong friendship, similar values and standards, common interests, etc.

Yes for life but if you've married the wrong person...

Depends on who you are, who your spouse is, how much value you place in your vows and given word. They mean remarkably little to some people. And they mean everything to others. I'm in 37 years with my spouse now.. wed for 32 of them. I promised him for life. He promised me for life... but I am his 4th wife! He blew through the first 3 in 10 years. I guess those were "in theory". 

yeah, don't you listen to the vows???

Yep - the promise you both make is that you'll be together til death. (Obviously not a legally binding promise, but that's the plan)

Vows usually include "...until death do us part".  Not good to get married planning for it to be over.