Here's the story with United:

They'll hold a Mileage Plus booking for two days without deducting miles from your account.

They'll hold a booking for 30 days after deducting miles from your account (there's no fee for redeposit if the reservation is cancelled without being ticketed).

But you need to be careful. Say you have 20,000 miles expiring in 1999, and lots more miles expiring in 2000.

If you first book a 25,000 mile award on a 30 day hold, they'll deduct 20,000 1999 miles, and 5,000 miles expiring in 2000.

If you next book a second 25,000 mile award,
they'll deduct 25,000 miles expiring in 2000