How long should I wait until I'll propose to my gf

I met my girlfriend in a foreign dating event and we have been in a relationship for a year and a few months now. Although, we are in a long distance relationship, I am confident that we are quite serious about each other. We talk every single day and we have a lot of plans for the future. From time to time, we talk about weddings, kids, and all that stuff but we've never really discussed it thoroughly.

Lately, serious thoughts about marriage have been popping up in my mind for some reason. This makes me curious as to when my girlfriend and I should get married.

I can see a bright future with my current girlfriend but it might be too soon to judge. How long should couples wait until the ultimate question is popped?