Is long-term chiropractic care dangerous at all? If you go say once a month?

It depends, if you are just going because you are hard on your body and just like an adjustment to "feel better" then thats great, no harm done.
However, if you are trying to maintain something that should be dealt with by a MD then ..... NO, it can be devistating to your body. If you have trouble with buldging discs, Sciatica,etc......then you really should be seeing a MD instead of using the bandage of a chiropractor.
There are alot of great chiropractors out there, but equally there are some really bad ones too. I have seen many of person in physical therapy that have suffered permanant damage because their chiropractor wasn't honest with them and recommend that they see a MD to consider what type of on going care would be best. Instead they opted to rake in a
"steady payment" at that patients expense. Unfortunetly there are alot of greedy chiropractors out there because it is easy to "get paid".

Best case scenerio would be to discuss it with a Medical Doctor, either your GP or an Orthopedist or Neurologist who has seen X Rays or MRI showing your spine and go with what they suggest for you.

Well, like anything, if you have a chiropractor that doesn't know what they're doing then I think it could have ill-effects on your health. I've been around chiropractic for quite awhile (both as a patient and as a chiropractic assistant) and the "once per month" treatment is often for "maintenance" care. There's nothing wrong with that. As a note, chiropractors "make their 'real' money" from "new" patients, not older "once per month" patients, so I wouldn't be too concerned about getting squeezed for money.

no.
every day is fine, every week is fine, every month is fine, every year is fine

It's not dangerous unless it keeps you from seeing a physician when needed. As long as you throw in a good dose of common sense there is no danger. There are some great chiropractors and there are some scam artists out there so be careful. I don't trust anyone who tells me I NEED to come in once or twice a month when I have no apparent problem. Chiropractors make their money on repeat business, if they can convince a few hundred people that they need to be seen every week for maintenence think of the money that brings in.
If it makes you feel good and you can afford it, go for it but I wouldn't believe anyone who told me I needed to be seen more often than once a month just for maintenence. BTW, my brother is a chiropractor and he is the one I got this from.