SRAM battery

Hello,

I bought a PCMCIA SRAM card a while ago for my Nord Lead. Now a few days ago the battery was empty and I had to replace it. I bought the card used, so I guess there was still a very old battery in it.

Now of course all the saved patches are gone (which is not so bad because there were only like 3 or 4 patches on that card  ), and now I'm wondering how long the new battery might last.

I think I read somewhere that they have a small backup battery inside the card, which means you can replace the main battery without losing the data within 20 minutes or so. My questions:
- is that true for all those cards?
- is this battery automatically recharged or can it be suddenly empty as well?
- how long will my new battery last, i.e. when should I replace it again to avoid data loss?