The honest answer for most homes
Never, on a schedule. A drain that works is not a problem waiting to happen, and paying to clean a healthy line yearly is money spent on nothing.
Where scheduled cleaning does make sense
Homes with original cast iron. Properties with mature trees over the lateral and a history of roots. Rentals where the same unit generates the same call repeatedly. In those cases the economics genuinely favor it.
What to watch for instead
Change. A drain that has slowed noticeably over a few months is telling you something. One that has been the same for ten years is not.
Why we are not selling you a plan
Because most homes do not need one, and offering one to everybody would mean charging a lot of people for something that does not benefit them.
How to decide for your own house
Two questions. Has it blocked before, and what is the pipe made of. A modern PVC line that has never given trouble needs nothing. A 1950s cast iron lateral under mature trees that has blocked twice is a different case.
Why the maintenance plan is usually the wrong product
Plans are priced for the average customer, and the average customer does not need one. If yours genuinely does, a specific schedule based on what your line actually looks like will serve you better than a subscription.
Rentals are the real exception
A unit that generates the same call repeatedly is a different economic case from a family home. There, scheduled attention often pays for itself, and it is worth basing the schedule on what the line actually looks like.
What to do instead of a plan
Get the line looked at once, properly, and find out what you are dealing with. A single camera pass tells you whether you have a healthy pipe that needs nothing or an old one that needs a schedule.