As often as you can be genuinely useful. For most service businesses that's monthly at minimum, weekly if you have real things to say. Consistency beats frequency: a reliable monthly note outperforms bursts followed by silence.
The real risk isn't annoying people; it's being forgotten. Lists decay when unmailed. Addresses go stale, and subscribers forget they know you, making your eventual email feel like spam.
Let the list vote: unsubscribes under ~0.5% per send and steady engagement mean you can send more; a spike says the last send missed. One useful idea per email is the whole formula.