The Paizo website is quoting the next release date as 23rd of March.
Seeing as the last one was out on their website on the 16th of the month, but not released by Big Finish until month end I have resigned myself to the thought that I will have to wait until April before I get to hear the next episode!
A bit off topic, but wasn't there originally a plan to do some companion chronicle style audios? Personally I think that would work really well telling stories with the four leads before they all came together.
Also a bit further off topic, but I see that there is a Pathfinder Adventures game for mobile devices (which is why it interested me) coming out based on the Rise of the Runelords adventure path.
The first chapter is free, so I plan to give it a go when it is released at the end of February.
And Big Finish actually telling us if, and likely when given the current treatment, there are delays. It's been at least two months since the last one, and the next one was quietly shifted to this month from last month.
Be sure to check out the indexes for the various Big Finish ranges!