For frequency switched observations, with FRSWITCH set, SWTCHDUR gives the time spent on each switch cycle. The scan must be long enough to allow a full scan setup. 15 seconds is probably the minimum reasonable switch time. For observations to be processed on Mark III correlators, there are additional restrictions on the possible integration times; multiples of 5 seconds are OK, as are some other times.
The frequency switching mode in SCHED is seldom if ever used so should be treated carefully.