Under Inbox > Automation, you'll find Rules. From here, you can create a new rule to prioritise a conversation if it hasn't been replied to within a certain timeframe.

When creating a new rule, select the event "A new conversation hasn't received a reply", and define the time period for which your rule should wait before kicking in (available as part of Inbox Premium or a Scale plan).

You can also select various conditions under which this rule should fire (for example, you may only want this to fire for VIP customers).

Then, choose the action Mark as priority. Save your rule, and you're good to go!