I would definitely call the vet, I don't know the details about the length of the heat. I assume you know all about pyometras by checking for a temperature above 102.5 deg, foul smelling and discolored discharge, and increased drinking and urination..this would be my biggest concern.
Heat cycles last a average of 21 days but that can differ from dog to dog. Too bad she was never spay. This could be caused by a number of things, an open pyometra being one but they are usually pretty I'll and do run a temp. You should probably take her into be examined, possibly get a culture done and a CBC to see if white counts are elevated. Ask for thyroid to be checked also.