Here's some interesting stats. From my admittedly small sample, it seems if you're going to get a positive response to your e-query, it's going to happen almost overnight. Of the five (count 'em! five!) positive responses to e-queries I received (2 partials, 2 fulls, 1 send-me-a-chapter), four were received within one day of sending the query; the fifth took 12 days. The overall median response time, positive and negative, was 4.5 days. About 70% of responses were received in a week or less. The longest took 43 days to dribble in to say fuck off. This is, of course, counting only those agents who bothered to respond.
Make of it what you will.