iCal automatically assumes that a new event will be an hour long, but you can manually adjust that into shorter minute intervals by using the defaults write command.
Here is the syntax to enter into the Terminal:
defaults write com.apple.iCal "Default duration in minutes for new event" 15
You can set the 15 at the end to any other number of minutes you want, then just quit iCal and relaunch it for changes to take effect.
This tip is from CmdComma, although if you copy/paste it from their site the command doesn’t execute properly due to the improper use of quotation marks.
If you’ve forgotten what defaults write commands you’ve used recently, check out this tip to query your command history.