How long does this pass last? Days or hours?
When I was there (May 2014), there was no time limit. You can only use the pass once at each sight, and one round-trip on the funicular/bus. I bought my at the tabachi in the train station on arrival.
I've not heard it referred to as a 'combo' ticket, maybe there's something else? I bought the "Orvieto Carta Unica."