Our team is using 5/16" OD 3/32" wall surgical tubing. We found a force calculator online for surgical tubing and found that most surgical tubing is most efficient in the 300% stretch range. We then based our design around that.
As for the lifespan of the surgical tubing we have found it is good for about 100 shots, but only because of the small nicks and cuts it had in in because of some sharp pieces on our robot. After replacing the tube, we compared an old piece and a new piece and found that the old piece only stretched an extra inch after about 100 shots.
We do know of the issue of temperature, being from Minnesota this is definitely on our minds as outside temps are just a little bit different from the inside temps...
So we just make sure the robot is properly warmed up before we start testing and practicing.
The force calculator can be found here:
http://www.primelineindustries.com/tools.html