In your head, it's not that hard.

100 hrs = 4 days 4 hrs.
72 hrs = 3 days

Of course, this is the time threshold where these upgrades can start to clear, but depending on the flight it's often not until much closer to the flight time.

Also, in your post above you mention the 25k miles + $350 upgrade, which implies you're flying long haul int'l (Europe, etc.). 500-mi. upgrades are not valid for these flights, so the 100/72 times above would not come into play.

If you're trying for an exit row, just keep a close eye on your flight seatmap at aa.com when you view your reservation, especially in the week before your flight. If one opens up you can grab it.