I guess if you want to keep the warranty and if money is not an issue then let the dealer do it at a service. Personally I simply don't trust dealers to do a good job so do mine myself.
With regard to your question:
A beer, unless you are a fast drinker is gonna be wasted - just chew on a gum and save the beer for later
Suzuki will likely tell you it is an hour and half job and will charge you as such.
As you are suggesting doing it I am presuming that you have an understanding of how to actually do it and are not a complete novice.
If that is the case then checking / adjusting the valves should take you around half an hour including all of the dismantling required - after you have done it once it will likely take about 15 minutes the second time as the dismantling will be easier.
The Inazuma requires (according to Suzuki) for the valves to be checked at every service i.e. annually. This I think may be a little 'over - the - top' unless you are doing a lot of mileage, but having said that it is such an easy job to do that I would do it annually just for peace of mind.
I don't think I ever needed to adjust mine, but did check them, and I often rode for distances at high rpm but it is better to be safe than sorry