How long does kegged Weizen (wheat beer) stay fresh?

The distributor is clearly wrong. If the system is properly balanced, the beer will neither go flat or get over carbonated. It will remain perfect. Beer freshness if relative. From the time you pick it up doesn't matter, what matters is the length of time from when it was kegged. for hoppy beers you have about 3 months, longer for other styles. Never accept a keg with a date that doesn't give you some time to have it on for a while. I believe most if not all imported kegs are pasteurized.

 

Your distributor has his head up his ass. Like Doug says, as long as there is gas on the beer it will not go flat. I could not figure why they would believe that to be the case. The level of understanding some professionals have when it comes to draft beer is mind bending.

Weihenstephan is pasteurized. That beer is really great and one of my favorites, but it is unfortunately not a great beer for draft. Still good, but not really any better than bottle. Because it is pasteurized you can expect it to keep for a good 4 to 6 months if not longer. The thing is, it never was fresh to begin with.
And remember, you can't be too choosy with the beer store. The store gets a delivery from the distro, and the best by date is fixed. They can't do too much about it except send it back to the distro. And the distro is pretty much helpless when it comes to delivery. They get it delivered from Germany. Not a lot of options here.

You will want to maintain around 3.5 vols, so the applied pressure is going to increase quite a bit. And if you are good, you should extend your choker to compensate for the added pressure. You are going to need the German A Slider coupler, and Oh the Joy of those things. Plan to get wet.
Cheers.

 

Distributor tells me that even with the CO2 tank it will start to go flat after a few weeks.

It would be interesting to have your distributor explain where the bubbles go. If you keep it in a keg with no leaks (very easy to do) it will stay properly carbonated for ... well ... forever.

I drink a lot of Weihenstephaner Weissbier (bottles) and it is my highest rated beer on BA. That said, the aroma does tend to fade after about 3 months, so it isn't ideal for anything long term. The keg pipeline from Deutschland might be all together different. You do know this will be an expensive keg.

You say you're new to kegging, so you really want to learn about "balancing" draft beer. The Weihenstephaner will be a little bit daunting because of it's high carbonation level. Recommend you learn how to read a keg carbonation chart (search Google) and read up on balancing. On this forum look for threads regarding "foaming", "balancing", and "help". You really want to get a good feel for all of this before you buy your first keg.

You will even have people of this board tell you to never raise the pressure over 12 psi because you will get all foam. That's like saying the laws of physics only apply some of the time . . . but maybe these people are hanging around your distributor(?). In Bavaria, you can be assured of finding this on tap with perfect pours. I keep a couple of Belgians on tap in the 17 psi range; they pour properly, taste great, and win gold medals. But you really want to understand a little bit about how those CO2 molecules are playing to make it all work. If you bog down with anything, feel free to ask questions.

 

I've encountered these at a few festivals I poured at... endured several highly public beer showers before figuring them out after a couple years. A very smart brewer told me the first step in switching out a keg is always to throw a towel over the whole works.