You only pay for the minutes of live chat between the time you clicked "Hire Expert" and the time either you or the expert clicked the "Quit Session" button. The length of the paid session is indicated by a timer  located at the bottom of the communication center (the chat window). Parts of a minute are rounded up to a full minute. 

Once you click "Quit Session" and depending on your initial selection, the session fee will either be charged to your credit card or deducted from your PrestoExperts account.