I see two options that could do this for you.
The first is by creating a custom field of date type that gets set to ‘now’ on first reply. Then you can create a new RT::Scrip using a custom condition and action. Where the condition returns true if the person corresponding is not the requestor and the ticket has not had the custom date field set yet. The action would then set the date field to now.
The other option is to load a RT::Transactions collection for the ticket and loop through the collection till you find the first correspond that was not by the requestor. If you made this a column map I believe you could then see it on search results as well.