Greetings all,
This is my first post so I hope you like it. I also hope that this hasn't been posted before (I couldn't find another post at least).
There is a very simple way to increase your flask duration. Once your last one is about to expire (e.g. you have 3 seconds left) just use your next one. It will show 2 hours but in reality your flask buff will now be about 1hr 40 mins roughly (didn't really measure it exactly). Enjoy!