I'm currently in a similar situation, I want to see how far and how quickly I can take my ideas, in my personal time, while keeping a comfortable day job.
I think there are a lot of variables involved, such as how much you value your current job vs. your passion and belief in the product you're personally trying to develop, as well as how much technical expertise you already have in the problem domain.
There may never come a time when I feel completely comfortable with the thought of losing the day job, but this makes the most sense to me right now.
"While we encourage clever code, and less mainstream languages (Hirewolves have a functional bias), these things are less important."
I think it would be nice if they told you why they favor functional languages. Do they have a higher percentage of openings with employers already using FP for things like concurrency, distribution, fault-tolerance? Clever coding and functional languages are great, but there is also the idea of using the right tool for the right job, which may not always be FP.
I think there are a lot of variables involved, such as how much you value your current job vs. your passion and belief in the product you're personally trying to develop, as well as how much technical expertise you already have in the problem domain.
There may never come a time when I feel completely comfortable with the thought of losing the day job, but this makes the most sense to me right now.
Good luck!