Prior to 2010 I was on the same board for 15 years. It was an old stiff cambered beast with kevlar in it so it surprisingly still rode well. I outgrew it in size though so decided to change. Now I still have the board I replaced it with, and it rides like the day I bought it (despite all the core shots and abuse!) but I chose to replace it with another style board as my tastes/abilities changed.
I also just bought a park/play board. I fully expect these three current boards to last me years. I would expect 100 days on a board would be at least what you can expect from one, unless you're landing tail happy all the time. I watched Timmytard ride my current board for a lap and cringed when he landed on the tail a few times, but the thing just flexed and took it like a champ! So far so good!
I knuckled a 30 footer last weekend and didn't crack the board.