This varies according to the pass in question:
BritRail Pass (England, Scotland, and Wales)
3, 4, 8, and 15 flexible within 1 month (‘flexi pass’)
3, 4, 8, 15, 22 days, 1 month consecutive
BritRail England
3, 4, 8, and 15 flexible within 1 month (‘flexi pass’)
3, 4, 8, 15, 22 days, 1 month consecutive
BritRail London Plus Pass
3, 4, 8, and 15 flexible within 1 month (‘flexi pass’)
3, 4, 8, 15, 22 days, 1 month consecutive
BritRail South West Pass
3, 4, 8, and 15 flexible within 1 month (‘flexi pass’)
3, 4, 8, 15, 22 days, 1 month consecutive
BritRail Spirit of Scotland: (these are only flexi passes)
4 days within 8 days
8 days within 15 days
BritRail Central Scotland (only flexi passes)
3 days within 7 days
BritRail Scottish Highlands Pass (only flexi passes)
4 days within 8 days