How many years later and the issue is still around...
I went through the disk cleaning exercise recommended by BookX, which did improve the amount of free memory, but made no difference to the export of my movies. I pulled every peripheral (printer, FitBit dongle, iPod USB, backup drive), and turned off wi-fi (never do use TimeMachine). No difference, even after three restarts. Likewise, the advice from I forget whose entry, suggesting I change all the slo-mo from 75% to 50% (a default speed). No dice. About the only improvement was that the export crashed into the heap zone after a longer period of time.
What's really wacky? I made this 88-minute film in three parts: two around 38 mins, one -- the credits reel -- only 8:15 mins. The credits reel exported to Large and HD1080 simultaneously in a couple of hours; it exported through MediaBrowser from a native project file of 1.16 GB. The longer parts 1 and 2 eventually made it into Quicktime as .mov files from native project file sizes of 320.3 MB and 438 MB respectively. That's right, the enormous 3rd part, mostly scrolling credits and some stills, one or two video clips, behaved fine.
I have made several 40-minute wedding videos that exported through MediaBrowser to iDVD without any problem. What the **** is going on with these two projects? And is this now going to happen on every subsequent film?
I am just trying Brian's workaround -- it went fine for 35 minutes -- up till five minutes ago, when, with 4 minutes left to go, the bar reset itself and now it says "about 2 hours." No, now it says, "about 39 minutes." I'm going to try shutting down wifi and closing Firefox. Not that I think it will help.
iMac running 10.9.5, 3.4 GHz Intel Core i7, 16 GB 1333 MHz DDR3 RAM, 2 TB hard drive with 821.35 GB free space