Your best would would probably be the /DU switch.
/DU duration
A value that specifies the duration to
run the task. The time format is HH:mm
(24-hour time). For example, 14:50
specifies 2:50PM. This is not
applicable with /ET and for the
following schedule types: ONSTART,
ONLOGON, ONIDLE, and ONEVENT. For /V1
tasks (Task Scheduler 1.0 tasks), if
/RI is specified, then the duration
default is one hour.
Windows XP: This option is not
available.
Quoted from MSDN. Why that information isn't in the TechNet Library page for schtasks is beyond me.