A full 6 hours
5 hours and one minute
1 hour (Almost no one does this, and you better be able to say you're Dutch or in a Dutch community. I have a Dutch name, I really should have considered this option.)
I believe there is one or two other options, but these are the primary contenders.
Most people seem to choose 3 hours or the full 6 hours. The most common reasons are
1) What your community does.
2) Your personal preference.
3) Your significant other's custom.
4) Your beit din may "force" you to choose 6 hours. I know of at least one who does, but others who "strongly encourage" it.
All of these are acceptable reasons to choose a waiting time. Personally, I chose 3 hours. I liked it better, and at the time, I had a medical recommendation that I needed to eat a small meal every 3-4 hours. In retrospect, I'm very glad to have this as my custom.
If you're considering basing your custom on a significant other, remember a few things:
A) That significant other might not be there in the near future. This is very common.
B) Then you'll be stuck with your custom, even if it's not what you would have chosen.
C) Many rabbis, if asked, do not require women to adopt all the minhagim of their husbands, including this one. It can be helpful to shalom bayis when raising kids if both parents have the same waiting time, but at the end of the day, it's a personal custom. Traditionally, women were required to adopt the custom of their husbands, but that seems to be changing today (in the mitzvot that are "personal" and technically don't affect the husband). Note that if your health requires a meal every 3-4 hours, that in itself may be enough for a heter for most rabbis. Of course, some will always insist that 6 hours is the only acceptable waiting period. You might want to know your rabbi before asking this question to see if you really want to ask the question.