My fax has been sending for very long
If your fax takes a very long time to send, it's likely that your fax contains a lot of pages. A page takes about 2 minutes to send, and that's if the fax machine of your recipient responds immediately.
Genius Fax also automatically retries up to 3 times in order to increase the chances for your fax to go through when the recipient's fax machine is not answering properly. So if you send a 10-page fax, it could take up to 60 minutes it the recipient's machine is really busy.
Note that you don't have to keep Genius Fax open for your fax to send properly. You will be notified once your fax is sent (or fails to).
What might also happen is that your connectivity wasn't very good and your mobile device didn't receive the notification that the fax was sent. You can always pull down on the fax list to refresh the status of your faxes and check that they have been delivered correctly.