Pipe viewer is for reporting percentage completion of files, pipes, and other file-like things. Nmap's completion times depend on network latencies, bandwidth, responsiveness of targets, and the features used to scan.

You can get a periodic status update with the --stats-every flag, like so: nmap --stats-every 10s, which gives an update every 10 seconds. You can get more verbose output from Nmap with the -v flag.