That depends on a number of factors. The primary cause of "skunk" is sunlight interacting with the delicate hop oils. So the more you can shield your brews from sunlight, the longer they'll last. Also, the higher the gravity/ABV, the longer they'll last.
With that American Lager, well, I don't know the OG on your recipe, but lagers, in my experience, are very quick to skunk out. So keep it hidden from sunlight, and don't count on it lasting as long as, say, the Guinness clone you're planning.
MOSS HOLLOW BREWING CO.
Aristocratic Ales, Lascivious Lagers
•Scottish 80/- •Sweet Stout •Roggenbier
.primary | bright:
98: Moss Hollow Soured '09 72: Oude Kriek 99: B-Weisse 102: Brett'd BDSA 104: Feat of Strength Helles Bock 105: Merkin Brown
.on tap | kegged:
XX: Moss Hollow Springs Sparkling Water 95: Gott Mit Uns German Pils 91b: Brown Willie's Oaked Abbey Ale 103: Merkin Stout
98: Yorkshire Special 100: Maple Porter 89: Cidre Saison 101: Steffiweizen '09 (#3)