I think it's different for every person. I think it's wise to wait until you're done college and in a decent financial position, because arguing about money can be a big issue, and you want to be able to finish college (if you're in college). That's just my personal choice, but it's different for everyone.
I think when it feels right. Although not to be rushed..you need a reasonable amount of time to get to know each other inside out & I guess waiting a while proves you can be together no matter what & that you don't 'need' to be married to do so.
when it feels right..
When you do not have to ask yourself whether or not to get married (in the same manner that you do not have to ask yourself whether or not to breath).