My daughter's Gemeinhardt step-up flute, which I bought for her in the 1990's, has never had or apparently needed such a service.
My Emerson, purchased at the same time, does need an adjustment, or possibly a pad for one key. That's it, after over 30 years. However, I hardly played it at all the past 20 years, so it has not been in continuous use. My daughter's flute has been used on average several times a month, but not daily.
Rather than pre-purchase a maintenance service, I think I'll just apply that money to a future service for my Emerson - as soon as I have finished paying for the Yamaha (a year or so). The Emerson is on stand-by now, and won't likely be used much at all.
I will, on average, play the Yamaha 1-2 hours a day over the next year.
My long-ago teacher never mentioned to either of us that after a year we need to have a routine service. She did a demo for each of us on proper flute care, including telling my daughter that her flute was NEVER to go out on the field with marching band!
How often do y'all send in a flute for service when you are not having any problems?
There is a local place I would use, here in Central Ohio, rather than send a flute away. I'll give that place a chance to fix my Emerson, and see if I'm happy with them. But that will be about a year from now.
Of course, if my wife wins the lottery or Publisher's Clearinghouse prize, I will expedite the service calls.

- Ted