Each manufacturer gives an idea of operating hours. I've found it simple to mark the packaging of each bulb with rough run time (say 18/6 from March to September) and replace well before manufacturer specs (bearing in mind company literature is always "optimistic").
Waiting for the bulb to pop is... another way of doing it.
Most T5 fall between 20-30k operating hours. At 8 hours a day you aren't even close, especially as rated lifetime hours are based on much shorter cycles (on/off), giving reduced bulb life.