The default behavior for Horizon is to retain everything, but we don't recommend people do that. We should change the default to match our recommendations. Note that this is a breaking change that would need to be made very explicit in release notes and would need to go out with a major protocol version.
What the default is might be debatable, but I would suggest 1 month as this is what all of our hardware prerequisites in docs assume.
The default behavior for Horizon is to retain everything, but we don't recommend people do that. We should change the default to match our recommendations. Note that this is a breaking change that would need to be made very explicit in release notes and would need to go out with a major protocol version.
What the default is might be debatable, but I would suggest 1 month as this is what all of our hardware prerequisites in docs assume.