Like this.... ?

#!/usr/bin/perl -w
use warnings;
use strict;

my (%log);

while (<DATA>) {
        my @fields = split ' ';
        if ($fields[8] =~ /(\d+):(\d+)/) {
                my $t = $1 * 60 + $2;
                $log{"$fields[3] $fields[4]"} += $t;
        }
}


for my $day (sort keys %log) {
        printf "%s workstation was in use for %.1f hours\n",
                $day, $log{$day}/60.0;
}
__DATA__
root :0 Fri Apr 7 06:00 - 03:00 (08:00)
user :0 Thu Apr 6 03:00 - 23:00 (08:00)
root :0 Thu Apr 6 06:00 - 03:00 (08:00)
root :0 Wed Apr 5 06:00 - 03:00 (08:00)
[download]

Running it gives me:

$ ./z.pl
Apr 5 workstation was in use for 8.0 hours
Apr 6 workstation was in use for 16.0 hours
Apr 7 workstation was in use for 8.0 hours
[download]

--roboticus