How far in advance do I need to purchase a Swiss Travel Pass and is the pass sent by email.

You can buy them at a Swiss train station when you first arrive. I don't believe there is a price advantage to an advanced purchase. But if you'd rather have them in-hand when you arrive, you can buy them from the RS travel store and have them mailed to you.

The consecutive-day versions of the Swiss Travel Pass can also be ordered to print at home, but the flexi versions within a month must be printed on special paper and delivered (or handed) to you. To be safe, allow about 5 business days for delivery before leaving home (although it usually takes 3).

Just bought the 15 day consecutive pass last week on line thru RS then
directed to Rail Europe. Paid online and had an e mail with ability to print
out pass 2 days later. Ready for the Matterhorn and Lauterbrunnen Valley
in two weeks.

I bought one recently on the day of my arrival at an SBB office at the airport train station in Geneva. It is the size of a boarding pass and came in a protective plastic sleeve

Zero time. You can buy them from and Swiss border or airport rail station when you arrive.
Alternatively, if it is a pass for consecutive days, you can buy online from the Swiss Rail website here: https://www.sbb.ch/en/leisure-holidays/travel-in-switzerland/international-guests/swiss-travel-pass.html
In that case you should get a PDF e-mailed to you within a few minutes which you print off and show to the on-train staff.

It is exactly the same price either way.