The Nmap 3.20 Statistics Patch adds the -c switch which guesses how much longer the scan will take, shows how many ports have been tested, resent, and the ports per second rate. Useful for scanning firewalled hosts.
You can download it from the following link: https://packetstormsecurity.com/files/download/31051/nmap-3.20_statistics-1.diff
Source: https://packetstormsecurity.com/files/31051/Nmap-3.20-Statistics-Patch.html