ALso too, without scanning the bon es, there could not be a set date. Fossles take about 10 to 15 thousand years to form, but this is also assuming that the jockeys bones are like that of creatures of Earth. who knows, they may be very mineral sensitive and could actually self fosslize in say, 100 to 500 years. ( not taking the weather as a condition too). But regardless, Alien eggs seem to be able to survive a long time untill they sense hosts nearby lol. And yes I know its a movie but you gotta throw real world logic in too =p