The fail safe, barring an actual spring breakage, is to pop the cap off the tensioner, and give it a left twist. The spring can be too weak to advance the mechanism, yet still, IMHO, be strong enough to prevent the adjuster backing itself off. The left twist would adjust the tensioner, just like a manual tensioner would operate-think of it as a semi-auto. At the same time, note how much you are able to turn the screw-should turn slightly if at all. Anything more than 1/8 of a turn spells trouble-the spring has weakened, in which case a manual adjustment can keep you going till new parts arrive, or broken (can be determined by turning clockwise, winding the spring). If the spring won't wind, you aren't riding anywhere, and you can congratulate yourself on having staved off valve destruction, unlike our forum unfortunates.
Theoretically, if the spring remains intact, one could manually advance the tensioner at oil change time indefinitely, or until the chain had so stretched that it's impossible to take up the slack anymore. Take comfort in the fact that cam chains rarely break-at least in my experience, though anyone could be the first to announce it here. I wouldn't hold my breath. I just know I plan to check and adjust the tensioner at each and every oil change from here to eternity-not complicated, and insures against stratospheric engine damage, as well as helps monitor chain health.
