It really depends on where you're flying from. We have been pretty lucky with AA miles from Miami, even fairly last minute.
As far as miles expiring, it depends. On AA, it's every 18 months now. I no longer have a credit card that I get AA miles on, so I have to watch my AA account sometimes, since we generally fly on FF tickets. Recently, I booked a rental car and paid a bit more, and got FF miles for it. Now I'm good for another 18 months.
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