Possibly the simplest way of all - open a terminal, log in as root, and type
tail -10 /var/log/messages
This will give you the last ten lines of the file (just increase the number if you need to see more), and should look something like this copy & paste from mine:
david@darkstar:~$ su -
Password:
root@darkstar:~# tail -10 /var/log/messages
Jan 22 09:35:13 darkstar pppd[2048]: primary DNS address
Jan 22 09:35:13 darkstar pppd[2048]: secondary DNS address
Jan 22 09:56:55 darkstar -- MARK --
Jan 22 10:15:51 darkstar pppd[2048]: Terminating on signal 15.
Jan 22 10:15:51 darkstar pppd[2048]: Connection terminated.
Jan 22 10:15:51 darkstar pppd[2048]: Connect time 40.7 minutes.
Jan 22 10:15:51 darkstar pppd[2048]: Sent 432975 bytes, received 2312561 bytes.
Jan 22 10:15:51 darkstar pppd[2048]: Connect time 40.7 minutes.
Jan 22 10:15:51 darkstar pppd[2048]: Sent 432975 bytes, received 2312561 bytes.
Jan 22 10:15:51 darkstar pppd[2048]: Exit.
root@darkstar:~#
Something else occurs to me - if you have the "dock into panel on connect" option checked in the kppp setup, you can simply hover your mouse over the applet, and it'll tell you the connection time.
Edit: Obviously you can only use the first method after you've disconnected