for how long is it safe to keep a whole, unopened coconut for decorative purposes?!

I doubt there is any danger. I think if it gets smelly or starts to leak, then you know it is time to toss it. Just keep it on something that can go in the dishwasher or the trash can. However, I suspect that it will eventually maybe go bad on the inside, but also slowly dry out and wither inside, and the outside may remain perfectly intact indefinitely. That's just a guess though. But go ahead and keep it, then if it starts crawling around, kill it and toss it. No harm done.

your lucky because of the hard shell you can keep it for no more three to two week maybe even thee but if was summer time i would say no longer the a whole week due to the sun... the sun will reflex in the windows and it can effect the color and the growth process.

Of course there is no danger. Are you SURE is hasnt been drained and preserved?