Contract for WP updates for client

I’m sure there are topics about this, but i couldn’t find them, so here’s my question:

What kind of deal do you make with your clients when it comes to updating the WordPress install on their websites?
(I’ve never worried about updating client websites, but right now i’m working on a website dat requires a bit more security, so i want to make some kind of maintenance contract for my client.)

– When do you update? After every new WP release (± 3 times a year), or on a regular basis that’s predictable for the client (e.g. 2 times a year on a set date)?

– How much do you charge? Do you charge by the hour, or do you agree in advance that it will cost 1 hour of work (for example), unless it turns out that a plugin breaks after the update?
(I want to charge a honest fee.)

Thanks for sharing your opinion!