Hello, we have a wood burning Lopi Liberty since 2010, use it for backup to our coal burner....so it gets about 6 hours per day use instead of 24 hours....anyhow...I wondered how long those air tubes hold up? we are thinking of getting some spares for the stove to have but are the air tubes an item that wear out sooner rather than later? I know these are relative terms...just looking for a rough guideline. TIA