Usually once a year, even if it hasn't been used all that much. I worry about high humidity affecting moving parts.

I bought a new Viking sewing machine, & it has required more maintenance than any of the vintage Singers I own. The older Singers I own are 15-91, 221, 301, 401, 403, Touch n sew, Futura, Touchtronic 2000, & Touchtronic 2005. Other than the last 2 on my list, I service them myself. I watched the video "Those Fantastic Featherweights:The Complete How-to Video for Maintaining Your Featherweight 221 and 222" & the "Featherweight and I- illustrated repair and maintenance book"
By David R. McCallum. The video is 3.5 hours total. Once I saw this whole video, & understood all I needed to know about all of my old Singers. Everyone who has an older machine would benefit from it; much of it is general mechanical machine knowledge that a repair person would know. I treat my machines well now, but the first 20 years when I was young, I NEVER took them for service. I didn't know I needed to unless something was broken;) Back then, I also never changed a needle that wasn't broken LOL!

I have my dh clean & oil after every queensize quilt. In between I clean (brush out) bobbin area every time I change bobbin. I oil once or twice a week on visible easy to reach areas. My singers are 36 & 52 yrs young. DH unscrews all screws and gets inside to keep them clean & lubedricated. I live in the desert. My sewing machine doctor said machines dry up faster here then in more humid climates.

this is my fear. my machines tend to stay working well, so i don't take mine in that often. in 40 years, i've had two inexpensive dsm's. they have never been serviced. i did have to take my emb machine in for a new computer board, though.

I service my mechanical machines myself. Since about the only thing you can do with the computerized machines is keep the bobbin area clean, I do that faithfully after every project. I only take my machine in if I have a problem that I cannot solve. After 20 years I took my computerized Kenmore Elite in and the problem was I had worn out the plastic presser foot! I would have NEVER figured that one out on my own!!

A Kenmore bought in 1961 has not been serviced yet, A Janome that I jumped out of time was re-timed and my 5yr. old Bernina just last month dropped its hooker or something like that. They all work fine.

A Singer bought in the '80's...eh?

Tallulah, the Featherweight, seems to be fine. I just got her last year.

Serviced? I don't know about serviced?

My original viking from 1984 was only to shop twice. I think it is still going strong, my daughter now has it. My Viking Designer One has only been to shop twice since I got it maybe in 2000. I don't know about the newer models, I think they have changed manufacturing location. I keep my machines dusted with the air spray, but that's it. They don't use oil.