How far in advance can a Swiss Travel Pass be purchased to be printed at home?
You should be able to buy them any time in the calendar year you intend to travel. It used to be that you had to buy them in advance as they weren't sold in Switzerland. Now you can buy them at the first Swiss train station you visit, so I would do that.
I just went to the relevant pages on the SBB website ( https://www.sbb.ch/en/leisure-holidays/travel-in-switzerland/international-guests/swiss-travel-pass.html ) and tried it.
Start date 14th August 2020 - success
Start date 15th August 2020 - "Your selected date of travel is not within the advanced booking period."
So the answer is 6 months.
There is however no advantage to buying that far in advance. The price never changes, and will not change until at least the annual timetable change in December 2020.
The only thing that changes is the exchange rate which could make it more or less expensive to purchase in Switzerland. But with the super strong SwissFranc and a US president who wants a weak dollar to make his exports more attractive I doubt we’ll ever see the time when the dollar affords us great buying power in Western Europe.
EUR at 18 month low vs USD. Now is the time. But in the case of Switzerland, CHF has been appreciating vs EUR.