The problem with this Poll is that you have to seperate EU and NA again. For me, an EU Player there is only one answer and this is: Check it monthly.
BUT if you are able to make a good setup where some servers get recked (like Abbadon) and others dominate every matchup (Deso+Vabbi) I would be fine with once a Quartal or even once every 6 months.
Short: Intervall depends of quality of the linking. Fair linking → less frequently; unfair linking → more frequently