The ironsgihts in the screenshot are shaderbased - a 2D image superimposed on your vision. An alternate version, which works better (no cross fading, true z-depth) is a model ironsights, where the gun model you see when non-sighted includes the sights as a detail, and that model moves up and into the centre. Thats how HI does it. (
www.planethalflife.com/hostileintent/ - they've got a pretty good Ironsights FAQ too).
As for the time, think about real life - how long does it take to look down the sights of a rifle? Or a pistol? just moments. Can you fire whilst your viwepoint is changing? Yes you can. Thats how it should be modeled ingame in my opinion. Accuracy stays in the unsighted mode until the shot after you've got it sighted tho - or increases gradually.
I think RMB is the correct key for it, as a toggle, rather than a push to sight type thing. You should be able to move while covering a target. SWAT teams and similar do it all the time.