There is no fixed answer to this. Every airport has its own versions of gate leases that range from longterm exclusive use deals to 30 day renewable use-em-or-lose-em deals to daily common use gate assignments.
Take Delta at ATL for example. The vast majority of Delta gate leases in ATL are "exclusive use" contracts through September 2010, which means that Delta gets to keep them till that date regardless of whether they reduce their schedule. These encompass all 8 T gates, all 30 A gates, all 34 B gates, 16 gates on C concourse and 7 gates on D concourse for a total of 95 of 98 outstanding gate leases held by Delta.
The only exceptions are gates D-33, D-35 and D-38 which Delta has on renewable 30-day contracts. These are "use 'em or lose 'em" gates and the trigger mechanism is an average of 7.3 daily departures from each Delta gate (excluding common-use gates on E concourse which are assigned on a daily basis), calculated on the first day of each month with a one-month grace period before default.
"The A340-300 may boast a long range, but the A340 is underpowered" -- Robert Milton, CEO - Air Canada