I cannot find an official answer but it seems you can Renew any time before it expires since you have to pay for it anyway
I have tried to renew and been questions as to why I wanted renewal with x yrs left on the passport.
In my case the US gov't wanted a 'frsh' passport w/ full term to apply their visa to.
The cdn gov't obliged
Others may need new pages.
But 'any time' is not true
remember, paying or not, it is not your property - the gov't will tell you 'when and where' when it comes to passports