I'd definitely agree July for a main/theatrical/2:30 trailer.
Given how they'll have more footage at their disposal now than for Part 1 at that time, I really hope to see the trailer campaign go back to blockbuster levels. Not having something with Avengers 2 would be disappointing, perfect time to debut a teaser (if they really want they can have something tiny like the Capitol TV teasers with Insurgent, then bring out the big teaser with A2) followed by main/theatrical trailer in July at SDCC (they better do a proper panel with maybe an extended trailer/exclusive footage?), then final trailer October 29, same as always, in time for Bond 24.
Having seen MJ1 I see why the trailer campaign was so tame this year...now they need to start and end with a bang.