Lightly but regularly used machines should stand up to 3 years. I don's see any need for grease points to be tented to more often, synthetic greases last very well. I don't know much about computerised machines, they seem to be clipped together, usually a solid aluminum inner chassis and very light weight parts. A lot of plastic that will run fine with out any lubrication at all. A lot of them are treated with some kind of lubrication. I don't know if sintered gears and bearings are that common anymore, they are said to be liftime lubrication free, or maintantance free. It's not completely true, and some recommend a particular stable and durable oil to be applied every few years. A lot of machines seem to be sort of clipped together and the parts doesn't run as tightly as some. They are more like a typewriter, and will move just as long as they are clean. I actually suspect machines in the £150 to $250 range is not really made to be taken in for repairs and full service, but they obviously can be. Some do a much more thorough service than others, and some are much better with particular brands than others. Trouble shooting and checking the computerised part is not easily done, only a few can. This part will usually not be done unless you ask for it particularly.
I have had my old Bernina in for a service. They will tend to the motor and fine tuning I might not be able to. I don't think I will hand in an old straigt stitcher, since there's a lot of expert info and advice available these days. It's well worth it though, full rewiring and trouble shooting is a lot of work, you have to be up to it and have the time. A DIY fixup takes a lot longer than an expert with a work shop.
For those who keep a mechanical machine, or vintage, we really should take time and sort out the basic. Most issues are about oiling, cleaning, and getting the needle in the right way. It can be a bit tricky, but usually its' not, just a bit cumbersome and time cosuming. A new bobbin tyre and motor belt isn't too much of a challenge either. They are meant to be taken care of by the owner, and it get's easy once we have done it a few times. A service guy will sometimes give you all the good tips and show you how to do it right. Not many talk to their customers today, we often just hand it in and pick it up.