For me, it depends on the MK opening hour.
For a 9AM opening, they let you in the gates 7:45-8ish, so you have an hour to stroll around Main Street up to the Hub. The welcome show is at the castle stage at 8:55AM. I will often leave Poly in the 8:15ish-8:30ish range, maybe earlier if I want to stop at the Main Street Starbucks (the line can be long).
For an 8AM opening or earlier opening, there’s no “hour on Main Street” time like with a 9AM opening. For an 8AM opening, I’d be leaving Poly no later than 7:30AMish. They’ll open the front entrance gates in the 7:45ish-8AM range.
Some folks may want to be more aggressive in their timing than what I’m suggesting above - I don’t like to do a lot of waiting around so would rather be more “right on time” than anything else. Others might be more comfortable allowing more time, even if that means sitting around.
Hope that helps!