[Lil' Lavery]

It really depends on how much you're currently stretching your tubing. Once stretched far enough, any material will begin plastically deform (permanently stretch) rather than elastically deform (stretch and then return to original length). Once you pass this yield point, each stretching cycle will cause a slight amount of permanent deformation, resulting in longer (and thinner) tubing, which will be stretched less in the future (thus resulting in less force). One solution to get the same amount of force would be to use more (or thicker walled) tubing, with each band stretched less. Another would be to periodically replace the tubing with new tubing.