By default, the software will allow 1 hour of no activity before you are automatically timed out of the software. If you find that this is hindering your work, you can extend the session to 24 hours by logging in via app.kashflow.com and put a tick in the box for “Stay signed in”. This is only available for users of the new user interface.
If you are currently on the old user interface, you can switch to the new one by following the instructions on this knowledge base article.