This script is run by cron every evening to make sure the time is set to 15 minutes:
#!/bin/sh
# Force times to a lower value than the usual default.
# 15 minutes
TVAL=900
USER=user
/usr/bin/dscl . mcxexport /Users/$USER -o /tmp/usertimes$$.exp \
com.apple.familycontrols.timelimits limits-list
awk -v TVAL=$TVAL 'BEGIN { flag=0 } /timeLimitSeconds/ { flag=1;print;next } \
flag == 1 { $1="<integer>" TVAL "</integer>"; flag=0} \
{print}' < /tmp/usertimes$$.exp > /tmp/usertimesnew$$.exp
/usr/bin/dscl . mcximport /Users/$USER /tmp/usertimesnew$$.exp
/bin/rm -f /tmp/usertimes$$.exp /tmp/usertimesnew$$.exp
Replace "user" with the login of the user you want to change.
Please let me know if there is an easier or more elegant way of doing this.