Kick... Yeah it's a kick, but all dependent on shot size. I would not advise going straight in with the 75 shot, start at 25 then tune. Run the car for a couple of weeks to see how the tune performs, do this for the 50 and 75 shot. So maybe agree a price with RS in the outset. Remember once the system is installed upgrading 25 increment shots will cost you £12 + any tuning time.
How long does the bottle last. I shall ask the same question to you. How long does a tank of fuel last? It's all dependent on your shot size and how often you use it. I can example a rough guide. The Nurburgring is 12 miles long, I used about 4lb in those 12 laps. So shall we say 30+ blasts, the longer you have your foot on the loud pedal the more squirt you use.
Here's a short cut to my build thread for the NOS
viewtopic.php?f=30&t=28423&hilit=Nitrous&start=90