Most growers will base their bulb replacement off of general usage or sight. A good rule of thumb is, Single Ended bulbs depreciate in 6-9 months and Double Ended bulbs 9-12 months. There are cost-effective candle meters that help time this swap more accurately.
Allowing your lamps to operate longer than the numbers above will result in, roughly, a 10-15% decline in output and yield. This loss increases with Air-cooled reflectors due to the glass in front of the bulb.