I've got a batch of cultured eggs going, when should I add the SB? Anyone have any favorite things to add to it?

Whenever they are done fermenting. Or in other words when they stop getting thicker. No big hurry to add the preservative like in meat or glands as the eggs will stop on there own when they are done. The preservative is just insurance to prevent bacteria from getting started after the process is complete. As far as what to add, add what you like with your eggs, if you get my hint. Makes a darn good eatable bait. Coons will wear out a hole trying to get more and coyotes will dig pretty hard trying to get to it.

Any rough estimate at how long it usually takes to stop getting thicker?

I think at room temperature I let it go about a week

My sons batch, 15 or so days. I'd have to check his book but Somewhere around that mark for it to reach this consistency. Time on preserving will depend if you use a liquid or granular preservative that would have to dissolve in time which would increase if it has to break down. You don't have to preserve like you would flesh bait and lures. Cultured eggs do well on their own in the right environment. jmo...As far as ingredients, you could add just about anything to cultured eggs and attract everything from skunk to canines.