The featherweight I add a drop of oil about every time I sew for very long. The rest of the working parts about every 2 months.
The Stradivaro I am not as good about remembering but try to oil about every 3 months. When sewing a lot, I am better about doing it regularly.
Mariah

It's actually a misconception that most new machines don't need oil. The manuals are poorly written when they say that they don't need oil. The assumption in the manual is that the machine will be going in for regular service where a technician will oil what needs to be oiled during the service. Unfortunately, when people read that it doesn't need to be oiled, they often think "Yay! My machine doesn't need to go in for service!" This is NOT true. If you're not oiling it, and I can bet many people don't want to get into the places where some of those oil spots are on their own, it NEEDS to go in for service.

Now, as to noise being the indicator of when a machine needs oil. We oil machines to reduce friction on metal to metal parts. This reduces wear and damage. When your machine is getting noisy, it's past the point of wanting oil and parts are wearing on each other. Don't wait for your machine to scream for help. Give it some preventative maintenance (oil) and clean out the lint while you're there! The lint wicks oil and to a certain degree grease away from where it belongs and causes premature wear that may not be covered by warranty.

I read in the manual every 8 hours. Realized that meant 8 hours of sewing. In some spots, that seems to be too much.

Oiling the bobbin area, Juki 2101, after 10 hours of sewing, there was a slight puddle of oil in the base.

I had a PQ 1500 and had to do my own repairs after a Brother Dealer cleaned it, and there were oil drips in the chassis.
These machines need a dipstick.

The Pfaff 2170 is sealed, but we were told by the dealer to put a drop of oil on the needlebar and takeup lever, and around the bobbin spindle.

My Brother sewing machine says not to oil. But, my Nolting Pro requires 4 drops on the red dots once every 8 hours of sewing and 2 drops in the bobbin case.

Since I have a newer machine (Babylock Crecendo) I do not oil mine.

My Computer Bernina tells me when it needs to be oiled and my other machines get oiled after each project.

I have two Bernina machines, same model numbers. The one in New Mexico needs oil sooner than the one in Florida. I am not sure if it is the lack of humidity that causes this or not. They do have an oil light to tell you when to oil but I do before the light comes on, usually after 6-8 house of sewing. They tend to get louder when the bobbin area is needing oil.

My repair guy has me oiling it every day that I use it. Now, do I??? I do when I am going thru at least a bobbin. If I just sit down and sew a tiny bit on that day, I don't. When I am doing some marathon quilting, I will oil every 5-6 bobbins. ( I just quilted 4 quilts with a total of 26 bobbins in 4 days) Many times it is just a bobbin or two and I clean and oil. I have a Bernina 630

I don't have to oil my Viking, but I do oil my 401A after I finish a project, and sooner if it begins to sound a little noisy.