As everyone says, there are loads of variables. I did about 12 miles yesterday, took about five and a half hours, but that includes pootling, stopping to look at an old windmill, taking a lot of photos, and a pub lunch...
But if I'd had to ride 12 miles direct, I reckon, with a favourable wind (or no wind) and relatively flat, then I might manage it in an hour or so.
Main thing, if you're hoping to do it regularly, like on a commute, is to allow plenty of time to start with, so that you're not running out of time and having to flog yourself and ending up discouraged and knackered. The more you do it, the quicker you'll get.