What is the purpose of using these?

These are used in a reflux still. The alcohol condenses on these and runs back into pot. This allows for the moisture that reaches the top of still to contain a higher percentage of alcohol. In short you get a higher percentage of alcohol by volume (ABV) from your still.

How long do they last?

About as long as a toilet. They are ceramic rings. They may get smooth but they will last, virtually, forever.

How many do I put in for a 1 gallon air stil?

1/8 to 1/4 cup or so, it's not exact. Lots of people also add pieces of copper.

how many bags suggested for a 2" x 3' column

The one pound bag I received contained 416 ml. Your 2" x 3' column has a volume of ‭‪1,853.328‬‬ ml. if you don't use any other stuffing, so you need about 4.4 bags. These are very small about .2 diameter X .1 - .4 length, so you need something at the top of your column to prevent them clogging the outlet tubing when you pour them in the column. I put some in a string tied jelly bag and slid it down the tube first for this purpose. They are dusty when they arrive, and must be rinsed thoroughly.