The answer is: it really depends! Not what you were expecting? Read on.
Certain styles of coffee - an Italian espresso for example - calls for a coffee to be rested for a full season before it's roasted. Other roasters want to accentuate the flavours that come from the fresh moisture content available in fresh lots. As with many areas within the coffee industry, our understanding of the chemical nature of what is happening in the bean from farm to cup, is constantly improving.
For a long time it was believed that green coffee does not go “off” and can be stored for indefinitely prior to roasting. It's true that if the roaster starts with sub-standard green coffee, no matter how freshly they roast it, or how incredible the skill of the roaster, nothing including freshness of the green coffee, can revive a bad foundation.
However, it is widely accepted by specialty coffee roasters that coffee stored in a bag for a long time begins to take on the taste of said bag ("burlap sacky" is not a flavour descriptor any roaster wants to hear). But that’s not all.
Understanding a further issue that storing green coffee over time creates, we must understand certain things about the roasting process:
During the roasting process, the moisture content of a bean breaks down, combines with acids and sugars in the bean, which caramelize. Without getting any more technical, this process is part of what gives coffee its flavour. If there is not enough moisture in the bean, this process will not take place, and the sugars and acids never caramelize, or the amount of caramelization is limited, instead "carbonizing", leaving the bean with that tell-tale “charcoal” or "tar-like" taste common in dark roast coffees. It is our belief that unless you intend to use our coffee to BBQ on, then this degree of roast is just not good!
Green coffee stored for lengthy periods of time will absorb and release moisture which has a long-term effect on the fibre-structure of the bean. As we live in the Great White North, we tend to have heat running for many months of the year. Heat - regardless of electric, or forced air, or air conditioning, will over time evaporate the moisture content out of the green coffee.
Artisan Roasters are relentless in protecting the moisture content in the bean and only with an intimate knowledge of the moisture content can an experienced roaster determine how best to roast the coffee. This is done by keeping coffee stored in ideal conditions, and only keeping on-hand what is absolutely necessary. It takes more time but it's worth it.
Too much (or too little), humidity will effect the moisture content, and over time (regardless of the humidity level), the moisture content of the bean will decrease, and we are back to the charcoal effect when roasting.
So in short, yes green coffee doesn’t go off in a way that would make you sick, but it does go off in a way that makes your taste buds feel sick!