Resort and Epcot Monorail hours change frequently, day to day and often week to week, so there's no "one size fits all" answer to your question.
If you are considering taking the monorail while there, make sure to pick up a current Times Guide in the resort lobby. That's the only way to stay current on the actual monorail operating hours for your days. The Times Guide is published weekly.
This website often posts the weekly Times Guide as well, it's worth a bookmark:
http://www.orlandoparksnews.com/p/weekly-walt-disney-world-times-guides.html
As you can see... the start times this past week for the Epcot line were:
Epcot Monorail Line
Sun/Tues/Wed/Fri/Sat - 8AM
Mon - 9AM
Thurs - 7:30AM (this is an Epcot AM EMH morning)
As others have suggested, from Poly it's often easier to walk to the TTC than take the Resort Monorail line around and transfer. Using this past week as a guide, taking the Epcot Monorail to make it there for RD or earlier would have been a valid option every day but Monday. If you're trying to be at Epcot 45 minutes early for a standard 9AM opening, I'd be circling being at the TTC right at 8 or shortly thereafter. Obviously I'm assuming no mechanical or operational delays with that advice.
BUT, monorail hours can and will change, so you can't always depend on the past. The Times Guide is critical to help guide your options while there.
Just FYI (and you may know this), but they start letting people in the gates at Epcot quite a bit before 9AM. You're then held at points further in the park until the real opening time.