Samsung are currently testing the latest Android KitKat 4.4.3 update on the Galaxy S5 and S4, while the KitKat 4.4.2 update could be out for some Samsung products soon.
According to SamMobile, Samsung are currently testing KitKat 4.4.3 on the Galaxy S5 (Exynos and Snapdragon models) and the Galaxy S4 LTE-A varient.
SamMobile also speculates that while Google might release the update at the end of May or in June, Samsung products will only receive the update about a month after Google's official launch.
As Google usually releases its Android updates to Nexus products first, and given that Samsung will be integrating KitKat 4.4.3 with its own software and features, SamMobile's speculation could be accurate.
Android KitKat 4.4.3 is expected to fix the multiple bugs plaguing users of Android KitKat 4.4.2. It is rumored that Android KitKat 4.4.3 will have more than three dozen bug fixes.
Samsung is concurrently testing KitKat 4.4.2 on a number of older devices, and has an estimated release date for the update.
Testing is completed for the Galaxy S3 LTE and Galaxy S4 Mini, and the KitKat 4.4.2 update could roll out in May 2014 and June 2014 respectively.
The Galaxy Grand 2 and Galaxy Mega 5.8 and 6.3 variants are in the final testing phase, and should receive the update in June 2014.
Meanwhile, testing on the Galaxy Note 3 Neo and S4 Mini LTE is still pending, and if successful, the update should make its way to both devices in June 2014.