I change all the time. I just started a new account (my fave so far

) and my house is currently a hollowed-out 2-storey tree with a few windows, a balcony, an epic staircase, a skyway to some other trees, and a rooftop viewpoint. I also have a long-ass tunnel (300-400 blocks?) that goes to a kick-ass harbour, which I may move to at some point.
I don't have a set mine, I just look around for caves. But I will get one.
Actually, my long-ass tunnel is like one block in comparison to dududf's. He's on the Escapist server, and has about 6 canals, each of which take literally 5-10 minutes to go down, and are about 4x4x6,000, with still water the whole way for boat travelling.