What I understand is that these things are like spices that you cook with - while old ones are somewhat effective - you will be better off with fresh. I would say after 6-9 months to toss it out and order new.
If money is an issue - I would probably use a 1 year old version of IF#2 rather than do nothing.
I just started a colon detox and had some old (6-9 months) #2 and used it yesterday. Today I started with my newer #2 and could taste the difference. The peppermint and herbal flavor was much stronger which I think is due to the fresh versus stale nature of ingredients.
I will keep my old #2 for external bites, zits and such as it seems to still be pretty darn effective.