Do tea bags ever completely decay ?

I don't even bother composting mine; I tear them up and then throw them in the garden. Even if they do have plastic content as the other post says (and that would explain why they're a little tough to tear, lol) exposure to light probably breaks them down as I see no lingering pieces of torn-up teabag from year to year.

Some do and some don't. Unfortunately, you don't find out that the ones you used don't until you find them in the compost a year later and have to sift them out.

Most tea bags have a certain percentage of polyethylene thread in them (anywhere between 5 and 50%, depending on the brand). This is to allow the tea bags to be easily sealed shut (two heated plates press the two sheets together, and the plastic melts, sealing the bag). 

Polyethylene is not biodegradable, so will not ever completely decay in our life time. The actual contents of the tea bag, though, along with the paper proportion of the bag itself, should compost pretty quickly. 

they might decay, i wouldnt use them if youve had them for yrs