10 years is the general rule. So if you're making the adjustments this year (2017), you can amend any/all returns from 2007 onward.
The entire process of adjusting multiple years can be done via your CRA My Account portal. If you are planning on adjusting more than one year, have all your info ready. Choose the option for "more changes" rather than logging in and out to do previous years separately. If you perform all of the adjustments in one sitting, you're less likely to face delays in processing.
