Battery or Alternator

Starting over the winter I started having to jump start my car every time I went out. I work seasonal 8 months out of the year and don't go out much in the winter when I'm off, so my car will sit for days (sometimes a week or so) without being used or started and I figured that's why I had to keep jumping my battery.
Now the warmer weather is here and I find it's getting worse, I figure it has to be either the alternator or the battery.
I've been told that if it's the alternator, it just goes suddenly and it won't keep the charge in the battery after I jump it and disconnect the plugs, but some say otherwise.
Any thoughts? Thanks in advance