For the most part universities in Canada start taking applications for the next year in early-mid fall. When you hear back could range from a couple weeks after you apply to late May or so. Highly competitive programs may not accept anyone until fairly late in the process, but many other programs would make offers shortly after you apply if you've got 6 grade 12's completed with decent marks.
If you are in Ontario and will be taking courses at a regular day school in the first semester, you may not get the information you need to apply to Ontario schools through OUAC 101 until mid-November (or you could get it a bit earlier).
