As others have posted, insulin keeps for a lot longer than 90 days if fresh to start with and kept cold.
Insulin must be kept cold but never frozen.

Here is an idea for prolonged storage of insulin, that will reliably keep it cool for a long time without any risk of freezing.
Obtain a very large, well insulated, top opening container. A dead chest freezer sounds good.
Buy an ice maker as used in bars for the bulk production of ice cubes.
Fill the dead freezer about half full of ice cubes.
Place the insulin in an insulated picnic cooler atop the ice, add more ice around the picnic cooler until the freezer is nearly full.
Whilst times are normal, top up the ice regularly.

When disaster occurs and the power goes off, it will take a long time for all that ice to melt. The insulin will be maintained at very close to freezing point but never actually frozen.

Better still would a home made insulated box with walls consisting of at least a foot of closed cell rigid foam, and a tight fitting lid.
Bigger is better, since heat gain is proportional to the surface area, whilst the amount of heat needed to melt the contained ice is in proportion to the volume of the ice.

A drain to remove melt water is needed, preferably fitted with a "U" trap to prevent loss of cold air.

It should be possible to make more ice from time to time by limited running of a generator, or by powering the icemaker from a large RE system. In winter of course naturally occurring ice can be harvested in many places.