Xero authorises us to look at your accounts for half an hour, but actually the process takes about two minutes. Xero calls this a 'public application'. Read more about how security is handled on the Xero website.
This is the screen you see as the process runs:
As you see, we fetch a range of data, starting with your Organisation details, and ending with Balance Sheets. For Invoices onwards, we fetch each month's data for the last 13 months.
Here is a video where you can see the entire process from registering with the site to loading all the data.