You can create one free format question on very first page of maxdiff excercise.
<input name="time1_r1" id="time1_r1" type="hidden" value="[%SystemTime()%]">
and one at last page of maxdiff.
<input name="time2_r1" id="time2_r1" type="hidden" value="[%SystemTime()%]">
Now you need to create one more free format page which can be calculate time.
<input name="time_r1" id="time_r1" type="hidden" value="[%Begin Unverified Perl
my $a=(VALUE('time2_r1')-VALUE('time1_r1'))/60;
return $a;
End Unverified%]">