I'll admit, I'm guilty of not checking my spare tire often. I probably check it once or twice a year. As far as I can remember, it's never had a significant drop in pressure but it can happen.
If tires went flat after just a few days as you say, we would have to fill up our regular tires with air every time we stopped for gas.
I keep saying call AAA because some people don't know how to install a spare. Put the jack in the wrong place and you are going to have a really bad day.