How and how often do I activate the license file?

The data of your license file is saved in your database. Therefore it's only neccessary to activate your license file once, no matter if you install on a network drive or on every single computer. You don't need to activate the license file on every single installation. Activate it for one database and all clients can startup Xpert-Timer.
Make sure you delete or rename the license file after activation, in order to avoid having Xpert-Timer prompt you with the "Activation question" on every startup.