Initially I felt like I ought to reply to every cold email, but quickly realized that that wouldn't scale (replying to cold emails took 2 hours a week, then 3, then 4..)

The litmus test I currently use is that if it seems like someone put more than 5 minutes of effort into an email, then I will reply. Sometimes emails fall through the cracks because I get swamped with work, but I'd say I reply to ~0% of "template" emails and ~75% of emails where the sender spent at least a few minutes thinking about the content and personalizing it.

Replies vary from "let's meet up" to "this doesn't fit my fund's focus areas" to "I'm not sure if the current version of the company is big enough for a VC business." I try to be honest if I have something constructive to say.