Do you have to guess the exact time or does the person who gets closest win? No way you can calculate this. The keys will not sink. The car will melt. That depends on the weather, which is not predictable. That said, if the closest guess wins, you should spend some time and find out how long a pile of dense snow usually takes to melt in the climate you have right now. I would try to dig up historical data etc. From the top of my head, I would expect it to take on the order of 6 weeks to melt down.
George