Currently, it is not possible to measure the duration or length of sessions within Heap. We have Average Time Between graphs as well as Time Engaged options that are similar, though these only measure time between events, not sessions.
You can use Heap Connect (upgrade required) to export your Heap data into an external data warehouse for further analysis. For more information and examples of what you can do (including time on site), see our list of Heap Connect: Common Queries.