Tricky question cause everybody has different opinions. Maybe on the homepage of the producer is an answer? I was lazy and waited 4 years *shame-on-me*
There is another post that is maybe interesting for you; had some more in the last weeks but without the "my answer"-tab I cannot find them without going back all the pages :o(
Greetings, Bettina

I don't know about the number of stitches, But i use my machine regularly and once I year i bring it to my dealer for a service and cleaning.

Not all machines show a stitch count-my Ellageo didn't-so I don't see how that should be a criteria. Once a year for cleaning and a look-see unless something goes wrong.

I have a baby lock ellegante, Our person who works on them says 5 million. I got chewed out because mine was at 21 million and I've only had it cleaned once. I told her I didn't know who was useing my machine, I don't get to sew that much LOL.

My service guy says about once a year (and one year for me is WAY more than a million stitches).

I've had a lot of problems with this very question. The salesperson at my dealer chewed me out and said it should take 2-3 years to put as many stitches on as I had in 9 months.
I emailed Brother and they told me it was more important to keep track of hours. They said every 300 to 500 hours. My machine is a 780D Brother and it counts stitches. I have 2,437,623 stitches on it now and had it serviced at about 1,600,000. I've put a lot of stitches on it since servicing....but no where near 300 hours.
They told me to change needles about every 10,000 stitches and to be sure to keep the bobbin case,etc. free of lint and fuzz.
I don't know why they put a stitch counter on it if it is not for servicing... but Brother told me hours, and could not answer my question about why they put a stitch counter on it.
MJDG