How long can a lithium-ion battery be stored before it becomes dangerously over-discharged?

I have a dustbuster that's been sitting unused for a while. I'm not sure how long, probably at least a year, maybe as much as two. It has a non-replaceable lithium-ion battery. It was probably fully charged the last time it was moved off the charger.

Is it safe to charge and use? I'm wondering how long it takes a battery to self-discharge to the point where it could be damaged and charging it could risk a fire. Is there any way to tell if this has happened?

The same goes for a few Bluetooth speakers that haven't been used in years. Do I need to throw them all away?

(Personally I wish lithium-ion hadn't taken over as the only rechargeable battery. It's good for cellphones where weight is critical and they're used every day. but for less compact devices and ones that don't get used all the time, I wish they would use NiMH batteries that don't have these safety worries.)