Normally whenever the party isn't in danger of dying from 2-3 hits is when a healer will DPS(so quite a bit in the older content, a bit less in newer because things are hitting harder now). It is all dependant on your comfort level, the tanks use of defensive CDs, and the DPS dodging AoEs. Most people, if you say you are new to healing, won't expect you to DPS if you're not comfortable with the class.
