When they puff or their chin they are trying to scare you off, its a defense mechanism. The bigger they look to a predator the better the chance of them surviving.
The only way to accuratly know when to change a UVB bulb is to invest in a solarmeter (solarmeter 6.2 are the best).
Typically however, if you have reptisun UVBs they are every 6months, if you have arcadia, its approximately one a year.