It depends on the ages, I think. I’ve noticed that older folks (30+) tend to get engaged way sooner than someone my age (early 20s) would. My boyfriend and I have been dating for over 3.5 years and we have no plans to get engaged/married at the moment, while others have dated for all of 1 year and are planning marriage.
It really depends on the couple. I am with you, I think 6 months is too soon if he didn’t know the girl as a friend for a while PRIOR to dating. I just think 6 months is not enough time to really get to know someone. 1+ year? That may be okay, if the people are older, independent, know what they want, and feel comfortable with each other and the relationship.
6 months though, I would be worried that he’s doing it out of infatuation, and that he doesn’t know enough about her yet to make an informed choice.
Again, there’s no real “set standard” but those are my thoughts.