Tank Questions

[hummer]

Hey everyone,
I have an extra 77Gal saltwater tank sitting around for about 5-6 months now without water. I have 2 questions:
1. How long can a tank sit for without water and still be good to use without resiliconing?
2. Is it possible to convert this tank to freshwater after it's been used for saltwater? There is still a bit of coraline algae on the bottom of the tank.

Thanks for any input!