Is there a rule of thumb for the decrease in value of complete sets without instructions?
If it's a newer set, missing instructions will usually just knock a few dollars off the price, even for larger sets. There are usually enough adult fans buying sets for parts that it's easy to pickup cheap instructions on Bricklink. For example, instructions for the new Millennium Falcon can be purchased for less than a dollar in new condition on Bricklink.

However, instructions and boxes for rarer old sets often vastly impact the price. For example, instructions for the classic Airport Shuttle (6399) are currently going for $40 dollars used and $100 new.

Basically, for newer sets the difference in price should only be a few dollars. For older sets where instructions may have been lost or damaged, the difference can be much greater.