How far in advance of travel do the Prem tickets
go on sale?
From tgv-europe.com:
"Prem's tickets are available between 90 and 14 days before the date of travel but are non-exchangeable and non-refundable."
PREMS tickets also sell out, so if you wait, they might not still be available. Buy them as soon as you can commit.
Ninety days, although it can be less approaching the European timetable changes in June and December, as the schedules are rarely finalised ninety days in advance then.