Plastering: Solutions to a cornice that has fallen off
What would happen if I just left that area unplastered for 4 months?


It would look exactly like it does in your photo, i.e. a bit ugly.

You could paint the exposed orange plaster to make it slightly less ugly.


  Is it feasible to plaster it oneself, and if so what tools should I use? 


Just take the cornice off the floor and reattach it with high strength grab adhesive and/or with nails. 5 minutes and done, and it looks less ugly than currently.

Depending on where your personal balance is for effort/ugliness you can either leave it like that, or try and use filler to hide the nail holes and sort out where the corners meet (they're never perfect) / make good the bashed corner.

Or you might even want to go and buy a new piece of the cornice which doesn't have a bashed corner, and mitre it to fit etc. But given you're considering just leaving it without doing anything at all, this is probably more effort than you'll find worth it.