Prune Scheme Command

I know these are the most common questions but not matter how much read the more confused i get.

What i need is simple, i want to keep the last 3 months of snapshots and delete any snapshots older then 3 months.

What would this prune command be?

Thanks kindly.

prune -a -keep 0:90

This will keep 0 revisions older than 90 days for all snapshots (-a)