Can I use a plastic cover for long time storage?

I don't see why not, but I guess the most important question is, what are you trying to protect it from? I think sun, rain, pollen, dust, etc. would be the most common answers, and the cover would protect from all of those even on a longer-term basis. However, plastic doesn't fare well under hot and cold shifts and probably wouldn't do well under intense sun. For example, one side of my house gets >12 hours of sun pretty much every day and it has really oxidized one of my vehicles' paint on the sun-facing side. To avoid replacing the cover every so often, you might want to consider a fabric or vinyl cover.