That said I would aim for as much as possible. Start small and aim for a month , then say three months and then perhaps onto a year or more. Longer term then it's essential to incorporate food production into ones lifestyle as it's simply impossible to store enough food for the rest of ones life.
This weekend we had a related and humorous little event. My parents house is now occupied by my wife's brother and family and we all went round for dinner. The garden is pretty large and although a fair bit of it is now grass my father and I used to dig it and plant vegetables . My wife's sister looked at the garden and said " but it's huge you'd be eating vegetables all year long " . There followed a silent pause before her father said " well that's the idea"
if any of you have read some of my other posts then you'll know I've mentioned my sister in law before and think she's a bit of a lost cause prepping wise. Worst thing is if anything big does happen I know we'll end up looking after her so storing as much as possible makes sense to us . On the subject of storing a large amount of stuff there is a good analogy from gardening which says that a soil is only as good as it's least constituent . Thus a soil can be very high in nitrogen but if it lacks say phosphorous then plants won't thrive. Equally having a years supply of food and only a weeks worth of cooking facilities is problematic. Keep all the preps in balance in other words.
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