Seriously, it should last as long as it would in bottles. After all, you are pushing it with Co2, and if you precticed proper sanitization, it will keep for a couple years.
I have a four year old barley wine and just finished a three year old spiced holiday ale. I tend to put kegs on tap at 3 months and finish them by 8-9. I've only had one ale that I dumped about half way through and that was one that I moved from California to Oregon.