Each process stores its start time, so it is fairly easy to find out when the process was launched. If you'd like to know when you started your PowerShell session, try this:
(get-process -id $pid).StartTime
And if you'd rather like to find out how long you have been working, feed the start time to New-Timespan which compares it against the current time and returns the time difference:
new-timespan (get-process -id $pid).StartTime
"You worked for $((new-timespan (get-process -id $pid).StartTime).TotalHours) hours now!"
"You worked for $((new-timespan (get-process -id $pid).StartTime).TotalHours) hours now!"
