The lamps cost $80 each, so they are a bit expensive. More important is the calibration swatch; it is intended to be replaced somewhat regularly since they will fade over time apparently.
My logic is that if you can get the lamp to calibrate in both channels, I wouldn't worry about replacing it too much.
But you should have a spare, so that you aren't out of commission for several weeks while waiting for one to arrive.
I have noticed that the UV channel will drift upwards as the unit warms up. If you calibrate it while hot, after a few hours of operation, it'll zero out just fine. Then, when you turn it on the next time, the UV channel will read approx -.07 on null, and eventually, the null reading will climb up to the point that it reads 0.00 again.