I'd backup the suggestions so far. The most likely point of failure would be the SD card. My own experience is that the SD card is fairly fragile. I had a power outage for just a second (New Zealand's electricity network is not that great even in the main cities!) and though the Pi would be able to restart after a power outage, in my case the SD cards failed dramatically. I had two Pis crash and get corrupted SD cards - I had to re-install the OS. My 3rd was able to pick itself up and reboot OK. Of the two that failed one had such a badly corrupted card that I was not able to put a operating system on it, it ended in the bin.
For a permanent fixture, you should consider that the power is likely to go off and come on without warning. Make sure your app will start automatically on boot. Also look at avoiding writing to the SD card - which will wear the card out (albeit very slowly). I'd imagine that an art installation wouldn't need to write to files or anything, but if it did then look at having your files in /tmp, which should be in RAM not on the SD card. You might consider ways of including a backup power supply. Perhaps solar cells or LiPo batteries.
Another approach would be to think about redundancy. Would your project be able to use two Pis, one that would cut in if the other failed? It could greatly complicate your project but for mission critical applications, that's a standard approach.
Sorry I've not included any actual figures as you have looked for, but maybe there's some information here that could help you think round the problem.