I always replace mine each year, to make sure it’s as “potent” as possible, but you’re probably fine using an older bottle—it’s advertised as good for 2000 quarts, and at 4-5 qts per day times 150 days (you don’t be drinking treated water every day of your trip) this means a bottle should, in all likelihood, last you at least 2 years.
That’s the theory, anyway. Nevertheless, I just say the hell with it and buy a new one—if the solution does weaken or break down as time goes on, and the iodine pellets start to break down, the stuff simply isn’t going to do its job, and I take enough chances with water as it is without using sketchy iodine.
Incidentally, for those debating using iodine, or hoping to do without a filter, I’ve used Polar Pure for years and have never, to my knowledge, gotten sick from water on the Trail.
Baltimore Jack