WIthout a HD cleaner, forever. I'd take the injectors to a good cleaning service and get them flushed and flow tested.

I had my 85 d50 sit for several years waiting on an engine ressurection (my original 2.6T transplant) and the old fuel caused no end of problems, and when I finally got it running right, I had put so much carb/injector cleaner and acetone and xylol thru it that the electical grommets came out of the external fuel pump and it puked half a tank of fuel on the ground. I had to tie the fuel pump back together with wire to get it home, dribbling fuel all the way.