The length of time it takes to backup is dependent on the number of files being backed up, the size of those files, and the upload speed of your Internet connection. Please keep in mind that for some broadband connections, upload speed is considerably slower than download speed.
Also, if the number of files being backed up is high (>500,000) or the average size of files is small (<1MB) then the speed of the backups will be limited by the number of file workers processing the backup. You can increase the worker count by editing the preferences for that Backup Agent.