It will (of course) depend on the time base of the timer you are using.
From my old training manual -
T - Timers
T devices are timer devices by default timers are either 100msec time
increments, 10msec time increments, or 1msec time increments
depending on the timer address. Most timers, depending on their address,
are non-retentive, meaning they do not hold their current value if the input
conditions stop conducting. In the FX2N, FX, FX2NC, timers with
addresses T246 and above are retentive. This means the timer hold its
value until it is reset. Retentive timers have a time base of either 100msec
or 1msec, depending on the address.
When a timer reaches the associated preset value the T device coil is
energized, the corresponding T device contact becomes active also. All
timers are 16 bit, meaning the maximum preset is +32767. Valid presets
are K values and D data registers. For a 100msec timer the maximum
time is 3276.7 seconds.
In GX-Developer, it is possible to assign a range of timers to be batterybacked.
This means the timers will hold their accumulated value in the
event of a power loss – provided the logic that drives the rung is batterybacked
as well. Otherwise the timer will be reset.
20 mins = 1200 seconds so use a timer with 100mSec time base with a value of K 12000