Note: I'm writing about mechanical switch keyboards here. Buckling spring keyboards like the Model M are similar, but are not identical, to the below. I could not find sufficiently good information about the Model M, so I treat it as similar to the below keyboards.
Polygon has a nice writeup of mechanical keyboards which notes that nearly all have a maximum decibel level of 56-57 dB. That's not very loud, but it is above the level of typical office noise. Remember that dB is a logarithmic scale, so an increase in 10 dB is actually a multiple of 10 increase.
They list the office they work at as having an ambient noise level of 46 dB. That's 1/10th the noise level of the mechanical keyboards. 50dB is the level of a quiet street, and 60dB is typical conversation - so it's somewhere in between, but close to "conversation" level.
As such, that's a good guide I suspect. If your office is such that someone having a conversation a desk away is noticeable, then it's probably too quiet for a mechanical keyboard. If someone having a conversation is not particularly bothersome, then you're likely in the clear.
But, as another answer notes - if you decide to bring it in, make sure to check with your coworkers pretty early on after you've brought it in (maybe a day later) to make sure it's not bothersome; and perhaps check again a week or so later. That way you don't leave them festering in annoyance with you.
Polygon dB level chart, slightly modified to include keyboards:
- Absolute silence: 0
- Rustling leaves: 20
- Quiet whisper: 30
- Quiet home: 40
- Brian's office: 46
- Membrane Keyboard: 47
- Quiet street: 50
- Mechanical Keyboard: 57
- Conversation: 60
- Inside car: 70
- Car: 80
- Motorcycle: 90
- Diesel truck: 100
- Power tools: 110
Microsoft keyboard dB level from PC World