Typically they will let me people in about 7:45, sometimes that gets delayed closer to 8am.
7:30 should give you enough time to get through bag check (assuming you are arriving by bus) and be ready to go. If you wanted to ride 7DMT or PP, then I would get there earlier to be at the front of the crowd, but I don’t think Tomorrowland/Space Mountain has as much demand from the rope drop crowd.
If you are planning to park at TTC, be aware that the express monorail often isn’t running that early and the ferry can be delayed as well (I think Disney purposefully starts these later since on-site guests are the only ones using EMH and typically are not driving their own car). There are several reports of people getting frustrated and missing most of EMH when trying to park at TTC, though you can always park, walk to the Poly and monorail from there but it’ll take much longer time.