I'd say it depends how often you use it (I wouldn't really care about this MTBF since I would imagine that the first thing to go wrong with an inkjet printer will be something physically wearing out due to overuse, rather than an inexplicable, random failure, though I could be wrong).
I use mine less than once a month so it's almost always off..
If you do switch it OFF and don't use it very much I'd be inclined to print a small coloured picture or piece of coloured text every 10/14 days just to keep the ink flowing and to stop it drying up and blocking up the print head.