Yes, if you store a sleeping bag compressed, the fibres will lose their loft (ability to expand back to their original shape and create those insulating air pockets). How long is long, well that is a hard question to answer... Depends on the materials, if it continuously compressed or not, how tight it was compressed, etc.

Due to this effect I do not use sleeping bags in any stored kits. It's far to waste full to keep my expensive sleeping bag compressed all the time. For longer periods of storage I would suggest wool or fleece blankets.