I know I'm allowed to stay 6 months in Canada as a UK citizen, but my travel plan is to stay in Quebec for a few months, then go across to the US for about 2 months, then come back for my return flight out of Montreal. However, I'm always nervous about going across the US border even though I've gotten through before. If for some reason I got turned away from the US, I'd then be staying in Canada until my return flight back which would be more than 90 days. So, do I just put 90 days as my length of time as that's what my travel plan is, or would it confuse them if I put something like 90-150 days?
I would just put the amount of time you are actually planning on staying in Canada. The important thing is that you don't overstay the date stamped in your passport.
Anything else is not really the truth, and that can get you into trouble. Suppose they look at your stuff and you have things associated with going to the U.S.- your credibility could be in question. And your fears of being refused entry at the U.S. border- do you really want to open that can of worms?
You will have to meet the requirements for admissibility, in the opinion of the CBSA, in any case, of course.