The length of incoming faxes are dictated only buy the amount of space you have remaining in your account. See options for space allocations.
 

For faxes of more than 20 pages, we recommend the sending party to split up the fax into multiple transmissions. Each fax relies on multiple phone companies’ ability to keep a persistent quality connection. While it is acceptable on voice transmission to have a small pop or pause, a fax transmission with such an abnormality would terminate the connection, losing all the pages in the transmission.