Tom, as far as losing interest, I have my eye on the big picture [cars/titan 90/ car hauler], and as I started each new color car 'completion' it almost felt like a new model, so it kept me going. I hope that makes some sense.
time savings? well, I usually take a month to build something [car/rig etc.], right now I am four days shy of 4 months on this project, so I guess I 'saved' 2 months.
to be honest, the other thing that got my blood really pumping again on this adventure was going to the Spring Thaw show this past weekend, seeing the 4 completed ones on the show table in surroundings other than my model room let me see them in a different light [literally], and that got my enthusiasm up as well.
I am quite certain I will never do 6 at once again, two maybe, 6?, forget about it