Unfortunately there're no specific log of user's login/logout times.
There are a couple of plugins available in the RDB Plugin store that may be useful, however:
The Login Session Management plugin will let you see who's logged in and at what time.
There's also the User Last Activity Report plugin that will show the time various activities were performed:
User Id.
Consultant Name.
Last item synced in RDB.
Last notebook item created.
Last CV created.
Last CV edited.
Last placement created.
Last job created.
Last applicant created.
Last client created.
Last contact created.
There is also the User History Plugin which allows someone who has the plugin installed to select a list of users and a date range. It will then show them a list of the following records that the user(s) have visited:
Applicant.
Job.
Placement.
CV.
Invoice.
Application Action.
Client.
Client Contact.
Notebook Item.
User Notebook.
Document.
Task.
The plugin will also show the date a user visited this record, and allow you to open the record directly from the grid.
By default it will keep a log of records for 24 months, but this can be edited in the plugin options to whatever the client would like kept (for a minimum of 1 month).