How long must you wait to get proposed to?

I have been with my man for almost 3 years now. We have been living together for about a year and a half and have been best friends for 4 years. We have discussed getting married SO often and i know he wants to but for the last year i have been waiting and waiting. I keep telling myself to be more patient but easier said than done! What are guys so scared of when it comes to marriage??