It takes quite some time. As it is the IT band and not the muscle it takes a lot more to actually stretch and make a difference. Stretch it whenever you can not just after running. If sat on couch try to stretch it with one leg on other knee. If waiting for the kettle put bad leg behind good and stretch. Basically just keep at it and you will notice an improvement.

Once it doesn't hurt any more, keep stretching, stretch some more then some more.

Again don't do the stupid thing like me and only stretch one side. Do both as it will just transfer to the other knee.

Think the explanation of the IT band being as strong as steel by detritus21 shows how long it will take.

Good luck with it! Really takes some perserverance to shift it.