Typically, an Ultraviolet lamp last 12 months or about 9000 hours. The lamp will stays lit longer than that time but after that year of use it’s ability to kill bacteria will slowly diminish. If the system is used seasonally, lets say for 6 months at a time and then it is drained and unplugged, you can use it for the next season for another 6 months before needing to replace the lamp again. There are some high end Viqua UV systems where the lamp will last two years but to make sure, check your manual to be sure.