I don't know, but the tensioners are only $10 to 20, so you should buy a spare.
I mounted mine on the pedal side, to even out the 2 chains, so it should last me a really long time.
If its necessary for your design build, just keep an eye on it to make sure all the bolts are tight, and make sure the bearing/bushing stays lubricated. When it gets some slop in it or wear in the plastic, just change it out to be safe.
I am just guessing, but I bet it will last 500 miles or more if not kept under a lot of tension.
If you ride long distances, see the threads on tool bags and keep a spare chain with you.