I've had my 920 for a few months but only recently got my FatBoy Wireless Charging pillow and JBL Power-up speaker. Wireless charging works brilliantly and begins as soon as you rest the phone on the charging device. I have a rubberised cover on my phone and it doesn't seem to make any difference or impede charging.
Exactly how long it takes will depend on whether the phone is on and what connectivity options you have switched on or indeed whether the phone is playing music etc while charging - i.e. how much power your taking out while you're putting power in, it's not an exact science. So, the best answer I can give you is my perception - wireless charging seems to me to be slightly quicker than cable charging. I've not yet run my phone out completely but after a full days usage, with the indicator showing about 20% remaining, I can be back to full charge in about an hour and a half if I'm not playing music.
As an aside, the biggest power drain I've encountered to date is Internet Sharing. I'm on 4G so sharing my phone data connection to my Surface tablet is faster than using wi-fi but it decmates the battery.... Just sayin'