I had a growler from a local brewery and filled it at bottling time. I made sure I sanatized the bottle and top. I did this to take a sample test before the three weeks of bottle time. Is this OK and how long will the beer last in a growler!
Most of the growlers that breweries and brew pubs use are not rated for high pressure of bottle conditioning. These are the ones with the screw on caps. They are designed to be filled with pre-carbonated beer. Some growlers with the swing top closures can withstand the increased pressure while the beer is fermenting before the co2 is dissolved into solution.
If you let them sit for about 2-3 weeks to let the co2 to dissolve into solution and they have not broke then they should be fine. Just be careful during the first week in the growler. After that time the growler should last as long as a regular bottle would.
I have used them myself without any problems but you need to be careful as the growlers are reused and may be weakened by continual use.
I occasionally will fill a couple growlers of beer right from the tap of my keg system to transport some beer somewhere. On occasions I might end-up filling more than I need and stick the full ones in the fridge. I find that the growlers don't hold their pressure well enough and the beer is somewhat flat after a week or so.
Now there are plenty of commercial breweries that put beers in growlers for distribution to stores. Their caps might be better than the "reusable" ones I am getting at my LHBS.
Otherwise, the beer should be fine for as long as a normal bottled beer would be.