thought this may be useful to someone.
I just did the old plea down the excesssive, but afterwards I asked the crown counsel about the length of time that has to elapse before they will consider a charter challenge for excessive delay.

She told me that there was no time period during which applications would be rejected, but eighteen months was the point at which they would "take a hard look" at whether it was worthwhile to proceed with the charges. I take this to mean you have a better than fair chance of getting off if it has been eighteen months from the date of the ticket to the first assigned hearing date (she told me this is the only timeline that matters).

my seat of the pants time/money/risk assessment told me it was not worthwhile for me even though it had been almost exactly eighteen months. However, if I was still a student or had extra time, I would definitely go for it. Even if you lose, it could end up being three years before you need to fight the original charges again. cop could have moved, etc.

hope this helps.