You can go back anytime you want as long as you maintain your "Residency Obligation". i.e. 730 days in any rolling 5 year period. This is particularly important when you want to renew your PR Card, which is valid for 5 years.
As an example, you land on January 1, 2016. Your 5 year PR Validity starts then (not 100% sure on this PR Card Validity may start from issue date. Seniors can confirm).
If you left immediately on the 1st, after completing your landing formality, you can come back to Canada before 1 Jan 2020, and then stay in Canada for the remaining period of validity on your PR Card. Any later, and you may be reported for not being able to meet your Residency Obligation. Please note that in any case, you will not be denied entry to Canada.
In case you do enter after 1 Jan 2020, you will not be able to renew your PR Card till you meet the RO again (i.e. meet the 730 day residence).
Hey Hot2Cold, I think you meant 1 Jan 2019. 730 means 2 years.
Without a valid PR Card, you will not be able to board a commercial transport to enter Canada.
Note: Your PR cards validity is not the same as your PR. What this means is that If your PR card expires, it does not mean you have lost permanent resident status. (See link below)
RO Link:
http://www.cic.gc.ca/english/information/applications/guides/5445ETOC.asp#appendixA