How long should the offer to marry your girlfriend?
It seems to me that the question of marriage is very individual. It's up to you to decide when to propose to your girlfriend. And the best thing in this case is to listen to her and her wishes. You need to understand how ready you and she are to start another stage of the relationship and build a family. For example, my brother registered with the polish marriage agency https://polishbrides.net/, met a girl there, and after half a year they got married. And my other acquaintances met for five years before signing and legitimizing their relationship.