It’s been about 53 days since 1.5 came out, and the average time between releases is currently about 76*. However, Rob mentioned during the AMA that they already knew what they wanted to do with 1.6, so it seems possible that this will be a short one. Also keep in mind that the Standard Deviation is hovering at about one month at the moment.
So there you go, make your predictions! My guess is August 10th.
*Really 79.4, but I’m using the median as a better measure of central tendency since there was a large initial outlier of 154.