Re: increse shelf life of biscuits
Based on the fat contents of your biscuit"s recipe composition, your biscuits fall on the category of hard dough (approx the fat contents around 20 to 30 % of flour). whereas for short dough, it is commonly agreed that short dough category contains much more fats, it can be as much as the flour (100%) or even more than flour it self. So, your product (some people called it biscuits) should be much more brittle, crisp or crunchy texture and harder than those with high fat biscuits (some people called it cookies), which is much more fragile and can easily go rancid.
Shelf life of your biscuit should be calculated based on its water contents (textural change) and should not be based on rancidity or flavor change.
You should have much longer shelf life if you packed them in low WVTR packaging materials. For example if your initial water content is less than 3 % and the crititical moisture content is 11 % (moisture content where your biscuits lost their crunchiness), you might easily get shelf life for 1 year or even 2 years, if your WVTR protection closes to zero. Besides, there should be not much flavor changes due to their low fats content.