That sounds about right. Johnny has expressed interest in doing extensive promotions for The Rum Diary, and there was even talk of a tour of a few college campuses. TRD comes out in late Oct., but there are several film fests in Sept so I'd imagine the promos to get sandwiched in between somewhere. It is a small film and he's producing, and he was also a dear friend of the author, Hunter S. Thompson, so I think he would want to be finished with DS so that he could devote his full time to that. Then, The Lone Ranger is also tentatively scheduled for fall, so I'd think DS might be done by Aug-Sept, but that's just a guess.
Edited to add- should've mentioned that Johnny's films with Tim have tended to shoot for at least 3 to 4 months. I think that Charlie and the Chocolate Factory stretched to even 5 months. Sweeney Todd went from end of Feb to May of 2007, I think, and Alice most of the fall of... which year, 2008?