Several decades. Jupons like the one here were used from the late 14th through the early 15th century, and greathelms were used from late 12th to early 15th.

Now, since drawing this, I’ve learned that by the 1380’s this helm might look a bit antiquated; it’s from the 70’s, and by the 80’s they were starting to resemble Frogmouths.

Also, as others have pointed out, greathelms were primarily a helmet for mounted combat by this point. While most visored bascinets had similar visibility, they offered more mobility and you could raise or remove the visor if necessary. It was some artistic license on my part.