How can I delete the storage used by my family on my iCloud account

You cannot, and should not need to do so. Once the Family Member is removed from Family Sharing, their storage usage will stop and any storage they were using will be automatically released by the system.

There is no particular time frame. But it should be fairly soon after being removed from Family Sharing.

The only indication will be the free and available space, since the actual files and content are not visible to you, you won't see them be removed, just the available space increase.