How long will iCloud take to backup and transfer video?

1. It really depends on your upload connection at the time. If your Wi-fi is boasting at least 2megs up, then you'd have to figure it will definitely take you a few hours to upload all that data, assuming that your speed is constant and uninterrupted without fluxing in speed.
2. It will be using your WiFi, otherwise your data cap would be hit almost instantly.
3. I got the 16GB for what it's worth...

Do you not have data caps on your wifi as well? Here in the UK we have so many GB we are allowed by the phone provider to use on cellular data, and we are paying an ISP for our wifi use via the desktop at home as well, which is equally, usually, capped. Sadly we do not move around the UK in a cloud of available free wifi paid for by others. 😟

If you have a wifi network in your house, sending any data around is free - it's just your wireless network, the data never leaves your house. If you're pulling stuff down from the internet or uploading to it then yes, that will apply to your ISP's cap, but you can synch your iphone wirelessly in your house as many times as you want and it'll cost nothing.

There are more and more people switching to Unlimited data tarrifs (at the prices that Three charge, what wouldn't anyone), and quite a few people use Unlimited internet at home too.

I have no videos and my iCloud backup is around 700mb, i still have 4.3GB of my free 5GB left, and I doubt i will ever use all that.

Using your WiFi network at home will is free yes, but it will use up some of your ISP's monthly data allowance.