For all fluorescent lights each start cycle shortens the lifespan. The current generation are designed to survive up to 20,000 starts before it dies. For earlier generations (less than two years ago) the number of starts was a fraction of this. Even so each cycle does increase the wear on the tube (which gradually reduces it's light output) and it's electronics. True the start only costs a few seconds of power but you lose lifespan/efficiency. At this point the cost/benefit balance is probably lower than 15 minutes but a lot depends upon the cost of the light - if you're using daylight balanced lighting this pushes the time upwards towards the ten to fifteen minute window.

If you are in the habit of switching lights on and off frequently look at the newer LED based lights. They use half of the electricity of fluorescents last in excess of 30,000 hours and have no limitations on switching frequency. (Ebay has possibly the lowest cost LED suppliers, but otherwise I've found that http://www.ledcentre.uk.com have a good range and customer support).