When to trash the bulbs..?

hey i use a son-t pia green power 600w and i dont know how old they are. starting 12/12 now so i doesent want to find out at the end that the bulbs was bad and i dont want to buy new if i dont hawe to. so what can i do to measure my ligths.? will a lux meter do the job.?
thanks for any replye

When i turn my light on, it get's bright, and cannabis grows, that's how i know there's no need to change it I'd personally only think about changing it if yield started dropping below an acceptable level.

my buddy owns a hydro store.. and he said. that u can let a grow light go till it burns out.. buttt every grow it will get weaker and weaker... depending on why u r growing thats how u know wen u change ur bulbs.. if u grow for selling purposes.. u change ur bulbs after 2 flowers.. or 2 vegs.. if ur growing for urself and ur cheap and u dont care if ur light is effecient then go till it blows out.. lol but my friend said most people change them every year.. or so.. but really if u keep ur bulb clean.. u dont lose alot of light.. but ive seen people with reallllyy dirty bulbs man.. u lose a ton just from a tiny bit of dust.. nevermind using the bulb long..

I go 6-8 months.
Can start to notice a major difference at that point.
I dont know if its just me but the bulbs seem to have a slight flicker after 6 months.

I have a 400 watt MH bulb and it looks as bright as the day I installed it , it will be 2 years in July. My 400 watt HPS I change after 9 months religiously. I have air cooled lights with tempered glass fixtures and I take out the glass after every grow, clean it well, wipe down the bulbs during dark period so not to interrupt the cycle and never touch any of the glass with my skin. I have a special pair of gloves I use so that no oil from my skin gets on there to attract new dust. You would be amazed how well they will maintain with a little TLC.