Gosh hard question. I have had some of my thread for 5 years and just keep it in plastic drawer container. I am guessing you mean packed away in storage shed for that amount of time.
I would imagine it would depend on where you live and if the temperature was going to be kept at a controlled number.
To me it would be hit and miss.

In clear or opague airtight plastic containers to keep dust ,bugs etc out.I still have thread that came with my first embroidery machine which is about 22yrs old ,that I used the other day & it sewed out perfectly.All my thread is kept in drawers or plastic boxes in cupboards away from light & dust.Summers are long here,with lots of hot days & high humidity.

I, too, store thread in plastic boxes. I have cotton embroidery thread in different thicknesses (30, 50 And 60wt) that I bought in the early sixties, when I had my first sewing machine and did a lot of free hand embroideriing. i used some pastel coloured ones last week as I love the matt sheen of it for some projects.without any problems.
I have Madeira rayon threads dating back to 1992 when I had my first sewing/embroidery machine and did not buy polyester until some 8 years ago - actually I got the complete Marathon collection and there are quite a few colours I never used. None of them give any problems except for a few large cheap cones I bought at a market.
So dust free, no sunlight and airtight containers work for me.

I store mine in airtight plastic containers

I got cotton thread that is 50 years old stored in a plastic box as long as they are kept out of direct sunlight you will be ok hugs

I would avoid the extreme heat of an attic or shed.

Interesting question. I keep mine in plastic drawers covered with plastic sheets to keep the dust out. They have been stored like this for the past 10 years. Before that in my previous homes they were in plastic containers in a glass cupboard but of the sunlight. Some could be as old as 18 years. Hugs

Thank You so much for your answers, you have been a big help. ♥