it initially read 1 hour, than 45 minutes then it went to 4 hours...at that point I was really desperate because I'm going to bed in less than an hour (I'm in Europe). Than, after a good ten minutes it went to 2 hours, than 1, than 30 minutes and so on...it took about an hour and 15 minutes to download OTA.
Currently installing...
What I mean to say it that it may actually take less than what it currently says.
At least that's based on my experience
I was doing it through iTunes so stopped the download deleted the file and tried again. On the device I wouldn't know....probably like imjoee mentioned restarting it would work.