Hi XXXXXXXXXX,
There's no particular time frame on how long the best man should talk, 5 minutes is a good time and will probably feel like a lifetime if you are shy. You want to be able to get your thoughts and feelings out, but you also don't want to put the guests to sleep, lol. You are the groom's "best man", the one he would come to if he had any problem, and the one he knows he can trust with his life. Share your thoughts about your friendship and your wishes for his new life (and wife). Write it down ahead of time if need be, or write down bullet points of what you want to say. Here are a couple of links that talk about best man speeches, maybe it will give you a few ideas.
http://articles.simplysearch4it.com/article/29311.html
http://www.presentationhelper.co.uk/best_man.htm
http://www.finespeeches.com/help/speech_planning.html
I hope this helps, if you have any further questions, feel free to ask, it costs nothing to click reply. If this has helped, please remember to click the green accept button.
Warmly,
Chase
#1 Just Answer Parenting Expert
Just Answer Parenting Mentor, Emotional, Behavioral & Physical Issues. Babies to Teens.