When is the right time to ask her out?

i have recnelty asked a question but worded it wrong, i am firend swith a girl i talk to her alot we crack jokes and listen 2 music togehter i wanna ask her out soon but when should i do it, could i do it on msn, at skl i rarely meet her outside skl. PLZ HELP ME skls nearly finished and shes movin away soonn