hw long does your macbook air last if you continously browse web and youtube ..im getting 9 hrs on wifi with youtube open i supposed to be getting battery back up of 12 hours as mentioned in apple website isnt it? ..i have macbook air 13"..

Note that http://www.apple.com/macbook-air/specs.html states "Up to 12 hours wireless web" and that it also gives the following information in the footnotes:

Testing conducted by Apple in February 2015 using preproduction 1.6GHz dual-core Intel Core i5–based 13-inch MacBook Air units and preproduction 1.6GHz dual-core Intel Core i5–based 11-inch MacBook Air units. The wireless web test measures battery life by wirelessly browsing 25 popular websites with display brightness set to 12 clicks from bottom or 75%. The HD movie playback test measures battery life by playing back HD 1080p content with display brightness set to 12 clicks from bottom or 75%. The standby test measures battery life by allowing a system, connected to a wireless network and signed in to an iCloud account, to enter standby mode with Safari and Mail applications launched and all system settings left at default. Battery life varies by use and configuration. See www.apple.com/batteries for more information.

9 hours definitely fits within the definition of up to 12 hours 🙂 - the “up to” is marketing wiggle room.

YouTube whether or not it is utilizing Flash is going to be harder on the battery than a static page like here at ASC and if it utilizes Flash even worse. Also your choice of browser has an impact. Basically I’d say you are getting good battery life.