The time spent by an agent on a ticket can be measured from the instant an agent opens a ticket. An automatic timer will appear in the footer of the ticket details page. 

Note: This feature is available only on the version of accounts that have Main Menu > Channels 

The timer inside the ticket reply editor will automatically continue from the time period recorded in the Ticket Details page, by rounding off to the minute value.

Post the agent reply the total time captured in the timer will be added to the Time Spent section displayed in the ticket details sidebar. Only upon performing this action, the time spent will be added. After a reply/private note is added the timer will be reset to 0 and start again.

Steps to Enable the timer

1. Goto Manage > Categories > Select the required category .
2. Click Edit Category -> Edit Advanced Settings> Time Tracking > Enable Autofill Time Spent > Yes.
3. Select the field Automatically start timer when an agent starts viewing a ticket? as Yes

Here is a quick overview with screenshots

The required auto timer setting is enabled

The timer automatically starts when the agent opens a ticket

Agent opens the reply editor to add a reply. The time spent is continued. 

Agent composes a reply

Total time is added