RotHC was released April 9th, 2013.
TfB NiM was released June 12, 2013.
S&V NiM was released July 10, 2013.
So both modes were released within 3 months of Makeb, while DF and DP are having to wait 6 and 8 months, respectively, for their NiM modes.
Within 3 months of Makeb, yes. However, if you count the time S&V was on the public test server, it would be more like 4 or 5 months, and if you count the time HM TFB was a level 50 op, it would be more like 10 or 11 months, until they had to wait until their NiM versions. Also, EC had to wait 7 or 8 months for its NiM version as well. So the wait for the Oricon NiMs is not out of line at all. Especially compared to EC and TFB. In fact, for the release of NiM DP NOT to be too early, compared to the level 50 ops when they were developed, a new expansion with a new planet and level cap to 60 would have to be released in March or April, at the very latest May.
As it stands, I'm predicting the new expansion to be sometime in either very late July, August, September, October, or November. August would make approximately 16 months of the level cap being 55; and the level cap was 50 for 1 year 3 months and 20 days. So, going by that, July 29 would be the date the level cap was 55 as long as it was 50. However, if they wait until November, EA Games and Bioware will have something to offer as their Black Friday big release. Personally though, I'm thinking in the middle of that range. Dragon*Con in Atlanta is the second-largest science fiction convention after only the International Comic Con in San Diego, and it will be held around Labor Day weekend (August 29 - September 1), and that event would be a great time for them to promote the expansion for release in late September or early October.