2 years, nine months.
Unbeknownst to DH, I'd actually set a timer. Early on in our relationship, I'd explained to him that what I wanted was marriage and children, and while it obviously far too soon for that to be on the cards for us, I needed to know whether it was something he saw in his future, because if not, I wasn't able to continue the relationship.
He turned a bit green, but stammered out that he could see himself getting married and having kids one day.
At the point I started the clock ticking. I knew already that I'd be very happy to marry him, and I decided 3 years was sufficient time for him to come to the same conclusion about me - if after that time, he still wasn't sure, he probably never would be, and I'd end the relationship and move on.
I never told him about this, because I didn't want that to be the reason he proposed. But I was very relieved he managed it within the time line anyway - I would've been devastated to leave him.
(I would've done though, because I wouldn't put my dreams and my fertility on the line for anyone).