I believe this new hairdo was only around for about three issues (and one cover) before Mary Jane took a nearly two year leave of absence from the book.
Amazing Spider-Man (Vol. 1) #64
Published: c. July, 1968?
Cover Date: September, 1968
“The Vulture’s Prey”
Writer: Stan Lee
Artist: John Romita and Don Heck
Inker: Mike Esposito
Letterer: Artie Simek
Romita, Sr. may well be my favorite Spidey artist, but that was not a good hairstyle for MJ!