By "how long," do you mean how long will the EMH be? If so, EMH is always 1 hour in the morning and 3 hours at night.
As for crowds, the last week of June leading uup to July 4 weekend is the most crowded of the summer, so it would follow that EMH will be more crowded than usual.
We were at WDW last June 15-23. The evening EMH were so crowded that it was not worth the time. We had much more success getting to all the parks before opening (EMH or regular). You can get a lot done in a few early morning hours before the big crowds arrive later in the morning.
Go with the flow.
We will be at WDW the last week of June, also
We go to all 4 Parks.
We use EE mornings at MK and MGM.
We will use late evening extra hours at AK and MK.
Go with the flow - get Fast Passes - avoid some of the parades (see them once - not twice) - Do rides during parades.
Get WDW rain poncho's or buy your own - it will rain in the afternoon