Alex, the problem is that no one seems to be sure
which film that is. As you probably know, Fotokemika does not have the equipment to make 400 ASA film, so all of their 400 ASA films are actually rebranded films from other manufacturers.
Up until last year you could be sure that KB400 is old Ilford's HP5 - but that batch was sold, AFAIK.
A little bird in Samobor, Croatia, told me that the 400ASA film currently sold is actually Agfa APX 400
In short, Marko's suggestion (12 min in Rodinal 1+50, at 20 degrees C) should get you in the ballpark - use it as a starting point, depending on your process. Develop one film for 12 minutes, and you'll see whether you need more or less development.
As an aside, I recently souped a roll of (new) Efke KB400 in Rodinal Special diluted at 1+50 for 15 minutes, and the film was a bit overdeveloped. The batch of Rodinal Special that I have actually has rather similar developing times as the "ordinary" Rodinal at the same dilution. BTW, Rodinal Special is actually Agfa Studional, if you need the dev. times.
So, I am inclined to think that with Rodinal 1+50 at 20 deg. C, your developing time should be anywhere from 12 to 14 minutes, depending on the contrast you need/prefer and your actual darkroom processing procedure, temperature precision, quality of water, agitation pattern, etc...
Denis