How often is everyone replacing their UVB bulbs? And who is still using coil UVB bulbs?

Coil UVB bulbs do not produce a consistent amount of UVB so they are actually a bit of a waste. Linear UVB bulbs will produce consistent UVB across entire span of the light, which at a minimum should 24 inches.

Most bulbs will start producing less UVB at around 6-8 months, but some can last longer than 12 months! How do you know when to replace it?

I got a tool that is pretty expensive, but for as many bulbs as I have to replace every year, this saves me hundreds as well.

Solarmeter Model 6.2R Reptile UVB Lamp Meter, Handheld Digital UVB Radiometer and Light Bulb Tester for Terrarium & Outdoor Activities, 0-1999 μW/cm2 UVB, Made in USA https://a.co/d/iu1BypE