How long before marriage?

Several years at least. Personally I don't think anyone has any business even thinking about marriage until late 20s or early 30s. People change a lot over the years and it isn't until that time frame people start to get a good idea of who they are, what they want and deserve in life, and what they want and don't want from a relationship. People are being coddled more and more these days and it takes longer for people to fully grow up. Because of that you seriously need to hold off on getting married until that happens.

When you do start looking at settling down, live togther for at least a couple years to see if you can even manage to deal with one another 24/7 like that. People that get along great when they have their space and become worst enemies when put into the same dwelling for a moderate amount of time. This sort of information won't be known until you actually get together and live with each other a couple years. After that, if you can still tolerate each other, then start planning a wedding.

Until you get all your ducks in a row, know you're FULLY compatible, including sexually, then hold off on the 100% commitment. If you don't you run the risk of being the 55% that gets divorce in this country. And just think about that. Flip a coin a few times to get a feel for how your chances are. Is that something you really want to risk without knowing for sure if you two stand a chance of making it?

It depends on the age and experience factor. Below 20, wait until after college and have been living with each other for at least a year. Mid-twenties, wait until both have jobs and have a stable income. 30s to mid-40s, been there done that and learned the lesson of not rushing things so in other words time shall tell. Mid-40s and up, some say time is running out while others are more concerned with not sabotaging their already formed families so in essence marriage comes harder to those with baggage but easier to those who have none.

Take your time, but I don't think there is a set rule to go by. Be comfortable with yourself, etc.,
be comfortable with the other, be friends first and make sure it is not just about paper and
what others say.