You can generate invoices automatically in the Xero app by creating the scheduler in Salesforce. 

To generate the scheduler, click on Setup, and search for the Apex classes button. Then click Schedule Apex, and enter the job name, select the "XeroInvoice" in the Apex Class field. Enter the frequency detail, Start and End Dates, the preferred time, and Save it.

Now the Invoices get created in Xero automatically at the time which is set in the Invoice scheduler.

Note: Make sure you link the Xero Contact with the object for which you are tracking time. Only then the invoices get created for the selected contact in the Xero app.

e.g: if you are tracking time for Projects in Salesforce, the Project should be linked with the Xero contact.