Dejan, from my experience of having the same problem. It didn't take as long as I thought it would. Your case may or may not be different. Just start playing the songs you know standing up, you may even wanna start by adjusting your strap so your guitar sits right where it would if you were sitting down. Right about the middle of the chest like tom Morello. As you get more comfortable standing and playing you may want to lower the strap down to where you like it. But by just playing more and more standing, you will find its very easy and make the transition very quickly.

[Edited by noticingthemistake on 12-28-2002 at 02:41 PM]