a boundary event is the correct element for a feature like this - to be more precise, a timer boundary event. When a certain amount of time has passed (the timer duration), the timer fires and can either cause an additional execution branch to form (escalation parallel with user task) or cancel the current task and proceed to a different one (escalation supercedes user task).The Activiti user guide has a pretty good documentation on it. How a subsequent email is to be sent depends on your choice of escalation style - in a parallel escalation you could choose a sequenceflow from the boundary event to an end event, and put an execution script listener on the sequence flow to actually send the mail.
