If you want to add the date you could use:
while [ 1 ];do date +"%x %X - `ping -c1 127.0.0.1|grep rtt`" >> ping.log;sleep 60;done
The example above waits for 60 seconds between pings.
The "ping.log" file would have entries like:
09/09/03 18:33:31 - rtt min/avg/max/mdev = 0.070/0.070/0.070/0.000 ms