More often than not plugs last a lot longer than what the dealer says they should be changed. However, there's a downside to keeping them in for long times. The plugs and the heads are not made of the same material and will rust/corrode together which usually results in a plug breaking off in the head when someone tries to remove it.
This is a common problem with phords.
Whether you change the plugs frequently or not, make sure you put anti-seize on the threads.