I was very surprised by how well they work - raided some my folks had been given (they had plenty!!!) and stoppered a couple of bottles of dev. 6 months later it was still *very* useable. I couldn't detect any oxidisation and it worked perfectly.
Longer than 6 months? Not sure - it will certainly be there when I visit AU again, so I guess I'll find out sooner or later, just how good they do seal a bottle.