When I cut them down, I have a can of paint with me to seal the ends as soon as the log is on the ground. If I make a new cut before milling, I seal that before I mill. I may be overly paranoid, but that's how I like to practice. If I have a client who wants something milled, I tell them to seal it right away. If I end up milling a log that's been on the ground a while, I seal it before I start. The problem is that, because of the grain of the wood (and the way plants have evolved to move water up themselves against gravity) the water can very easily wick out the ends. It wicks REALLY fast out the ends and hardly fast enough out the sides (esp. when the log is in the round) so the ends dry way faster than the middle. Drying = shrinking, so the ends end up shrinking faster then the middle. However, the structure of the middle tends to hold it together in its original diameter while the ends are shrinking. That means a split must occur.
I don't know the actual time it would take before checking starts to occur (though someone on here probably does), but I would just as soon not take chances.
All that said, I know of a huge red oak that's been on the ground for probably a couple of years with no seal which I may get a chance to mill. I'll probably still try it. However, I expect to lost a foot of length at least on each end when I end up using the lumber.
