I guess you are talking about Mountain Lion not mounting Lion which is the latest release of OS X. You need to connect with your service provider to check the Internet speed or run a speed meter so that you get confirm about the speed. Otherwise it will take just less than 30 mins to get download from Apple Store.
Well, that depends on internet speed. If the OP is downloading then entire 4 GB file, then it may very well take much longer than 30 minutes. But unless there is a problem, then anything beyond 2 hours is max time.