Well, Grace, first of all is make sure you are aware that best friends could be ruined by relationships, but if you feel like there is more to your relationship and you feel like it could be more it may be time to move up from bff to bf/gf
The short answer is you don't know! But if you have been friends for a while and you share common interests then thats a good base to start an intimate relationship from. Obviously there also needs to be physical attraction but I'm assuming thats definately the case already, so when do you take the plunge, well it will probably just happen naturally one day a friendly hug will become a first kiss and from that moment on your whole relationship will change and become something completely different. Of course there are no guarantees in life just because you were good friends doesn't mean you will be good together as partners, but thats the gamble you take when you start dating a friend because unfortunately if the relationship doesn't work then the friendship tends to end as well. Good luck.
I had the best guy friend ever...he always listened to my problems...and the whole drama bout my crushes lol..i knew i liked him but i didnt want to admit it cause i was scared...even wrose he had a girl friend...well there relationship wasnt working so they ended up breaking up...well enough of my story...whats important is that u follow ur heart and slowly find out if he likes u back..i asked my bestie straight out if he liked me..but i dont know what type of person your bestie is so i cant say what reaction he will have...and well i ended up dating my bestie and we are still together...and well good luck grace(: