A long time, even if the plastic bottle starts to suck itself inwards at the sides it will still hold your alcohol, and alcohol in the form of hard liquor does not go off (too much alcohol for bugs to grow), the flavour may alter after a few years though.

I suppose you can hold it a very long time since some hard liquor is sold in a plastic bottle. But I must warn you, these are the cheap sorts and they should be avoided, if you are interested in quality. Here some page talking about "plastic bottle" (search this keyword on the pages): "I've bought vodka that came in plastic bottles before so I think it's ok to drink There's plastic, and there's plastic. Glass is always better. Plastic *designed* to contain hard liquor (such as the heavy-duty plastic used for vodka that shipped in a plastic bottle) will work acceptably well, but it's not as good as glass." "Crappy plastic designed to be used for water will not always stand up to hard liquor. Unless you are a chemist who knows the difference, don't keep good vodka in a plastic bottle for more than a few hours. poland spring bottles won't ruin the vodka gallon milk jugs will (over a few years, not even hours)" http://www.tribalwar.com/forums/archive/t-382410-p-3.html "I find cheaply distilled liquers [give me headaches]. Things like the Popov Vodka at the grocery store in the plastic bottle." http://forums.firingsquad.com/firingsquad/board/print?board.id=offtopic&message.id=55634&page=1&format=page "And avoid vodka that comes in a plastic bottle." http://www.relevantmagazine.com/boards/showthread.php?t=8353 "and damn near anything in a glass bottle, you'll learn that most of whatever comes in a plastic bottle tastes like it could de-grease an engine. I'll still drink it, but i won't enjoy it as much." http://forums.worldofwarcraft.com/thread.html;jsessionid=1523B66F951CDE6289C82B3251F7AC9D?topicId=106731553&sid=1&pageNo=2 "booze (no beer or pop, use plastic bottle for hard liquor)" http://www.kootenaymountaineering.bc.ca/equip.html

In the freezer I store a Plastic Margarita tub with spout, Margarita's mixed with the liquor in it and serve Margarita Slushes! The spout doesn't work but you need to open and stir a little anyway to get equal servings. As long as I have room I store all my liquor (mostly glass bottles) in the freezer but have never had any problems with plastic containers.

i kept bacardi O in a used plastic water bottle for over a year and a half...it still tasted fine and i didnt get sick

For me, hard liquor means whisky, and whisky means Scots whisky. Most of the cheap blends are sold in plastic, and are probably good for many months. I wouldn't know, as whisky never stays long in my liquor locker. Single malt, double distilled whisky is always sold in glass, and will remain pristine for decades. But if you're going to STORE it, you ought to buy it in the cask; thus it will improve with age. Whisky in glass never changes.