Updates usually take about 4-5 weeks. So, 0.28 should drop by the first week of May.
This "very short" stuff is more complicated than you seem to think. For instance, many players have complained, starting with 0.01, that a walkthrough was a necessity to play this game. A "Hint" function was introduced in 0.27 which made it possible to now actually play without a WT (we'll know how well this works in future updates). Also, the excess grinding of Parts 1 and 2 has been dispensed with. These improvements, and a few more such as an Android version for the first time, take dev-team time away from providing more game content. An alternative would be to provide an update every 2 months. Few players, I'd imagine, would want that even if that meant an end to thin-content updates.
As someone who has played this game since 0.01, the pattern has been that an update that most players would consider thin is very usually followed by a much more substantial update.