I waited 25 minutes last Friday having arrived off the 283. I was probably the first Economy PAX off the plane and overtook a number of people on the long walk to immigration.

If you're further towards the front, knock a bit off, if you're further back, add a bit on.

Be aware that if your are a UK citizen travelling on an ESTA, and have had at least one visit to the US on your current passport since 2008, you can use the e-immigration gates. I only found this out AFTER being manually processed, and would probably have sped things up.