molecular biology - How long can E. coli stocks be stored at -20°C?

I'm volunteering for a biohacker lab - biocurious in Sunnyvale. The have a pretty good set of equipment - gel boxes, incubators, but they don't have a -80°C freezer yet.

I'd like to set up some glycerol stab stocks of some E. coli strains, but i'm told even untransformed (i.e. no plasmid) bacteria will get funny after a while at -20°C. It would also be good to hear how long you can keep a plasmid transformed strain at -20°C.

Can anyone be specific about what happens and how long it takes when you store them at -20°C for long periods of time?

I have done a little reading for competent cells and I guess they stay active for about 4 days without a -80°C freezer.