I use a bleach of similar strength (22g ferri, 25g potassium bromide in 1000ml water). I use it full strength, and store it in an amber bottle out of the light. My current batch has lasted more than a year, and in my eyes still performs as intended. I've seen a batch take on a greenish tint, at which point it was tossed. I've never seen a chocolate colour, like you've mentioned. Something is wrong there.
Perhaps the other option you could do is make 10% solutions of both Potassium Bromide and Potassium ferricyanide, then mix the bleach up from that. They may last longer divided.