Currently sleep mode is activated after four hours of inactivity. A quite reasonable time I think. So instead of fiddling with the sleep setting I added the following line to the crontab of our server:
0 7 * 1-6,8-12 1-5 echo | lpr
This line prints an empty page every morning at 7 each workday, except in july when the office is closed. Thus the printer is calibrated and ready to go when people start arriving at the office around 8 in the morning.
Our server is an Xserve running OSX and for this to work I had to install the printer drivers on the server. Also setting the printer to default helps I think (though I didn't have to since we only use one).