You can't really tell when the UVB runs out, without a solormeter. The bulb will still produce "light", but the UVB spectrum gets dimmer. Us humans can't see the UVB spectrum with our eyes. That's why it's important to remember when you started using the bulb and change it every 6 months (zoomed), or 1 year (Arcadia). If you can afford one, a solormeter is a really handy tool.