It is a two part process - you define schedules and then apply that schedule to a policy. To define a schedule you use the following command:
set schedulers scheduler name dayOfWeek start-time stop-time
So for example if you want a schedule to be active every day from 8 to 5 you would do:
set schedulers schedule allow-web daily start-time 08:00 stop-time 17:00
You then create your policy and apply the scheduler to that policy. The policy is then "active" during the scheduled time period.
set security policies from-zone trust to zone untrust policy allow-web-out match source-..........
set security policies from-zone trust to zone untrust policy allow-web-out match destination-.....
set security policies from-zone trust to zone untrust policy allow-web-out match application ..........
set security policies from-zone trust to zone untrust policy allow-web-out then permit
set security policies from-zone trust to zone untrust policy allow-web-out scheduler name allow-web