Last October EmailLabs measured "read time" across millions of messages sent by their more than 300 clients to permission lists (i.e. people who had signed up for and were expecting to get the email). Their intitial results showed that on average readers are spending 15-20 seconds on each email they open. Read more about this on MarketingSherpa (free access until Jan 23).
eINFO - a collection of articles and studies about email marketing.