Compressing synthetics over and over will gradually break it down. Same with down, but down has a longer life span (I've heard of 30 year old down bags) if you keep it clean and store it properly between uses. I'm given to understand that synthetic, used regularly, may last a few years.

My synthetic and my down quilts hang in a closet, loosely. I'll clean them as necessary - haven't noticed a lot of grunge buildup yet, wearing clean clothes at night to sleep in seems to cut that down considerably.