Hello how frequent do you need to spray each item or is it once per season?

My husband sprays each outside ceramic at least 2 times before placing the item outside. Then he only sprays again in about 2 to 3 years depending on the sun exposure,

You can definitely spray it in a plaster statue, but just be aware that it leaves a cloudy film that's rough to the touch and next to impossible to get off.

Can this be used on canvas chair covers?

I used the SunGuard on my outdoor metal decor. For my outdoor chair pads and umbrellas (fabric stuff) I use Scotchguard Outdoor Sun and Water Sheild, it helps reduce UV fading and repels water.

Has anyone tried it on greenhouse plastic sheeting?

Yes it may work, but I don't believe it would be cost effective. The can costs about twenty dollars, it covers 25 sq. feet. The directions recommend applying every 4 to 6 months. You probably would be better off using a good UV resistant plastic. Bob