Carbing Cider...too late?

I made a 3 gal batch of cider 5 months ago and have racked it a couple times and it has cleared perfectly. I have it conditioning in a 3 gal carboy under airlock in the basement and was wondering how long is too long before priming will not work (lack of viable yeast). I would like to bottle in another 6 months or so but don't think that priming will be effective without adding yeast and was hoping that wouldn't be necessary. Any information would be much appreciated.

If it's cleared I would bottle it now and age it in the bottle. That way you won't have to worry. However, you can leave it in for another 6 months if you choose. In the end, syphon off enough to fill two bottles. Try adding about one tsp of priming sugar to each bottle and give it a good month to carbonate. If it shows no signs of carbonating, then you could try pitching some more yeast and bottle.

Chances are good that the cider completely fermented out, and is around 0.998. Even if you add more yeast to this, there isn't any sugars left for them for munch on. You would have to add in some sugars along with the yeast if you want to carb it in the bottle.

I made several batched this year, and when I was at the point where you are, I crash chilled it to make sure all the yeast fell out of suspension, then kegged it along with some dememara sugar or honey, and force carbed it.

It is extremely difficult to get all of the yeast out. Not something you need to worry about even after a year.