nmap -sP 192.168.0.0/24 | grep ^Host | sed 's/Host //g' | cut -d " " -f1 > ips
Then, you can use your openvas server in batch mode
openvas-client -q 127.0.0.1 9390 user pass ips "report-`date`.html" -T html
viola, set a cron job and you are good to go. Maybe you would then cat the report through sendmail to yourself or move it to another server.