Idophor has a 20sec required contact time, this includes the time it is wet and dripping. A small amount of solution in the carboy, shaken up to cover all the interior surface for a few seconds, then drained should be sufficient.
Same goes for bottles. They sell a device that squirts a small amount of sanitizer into the bottle. You can do this instead of filling them all the way up, soaking, and draining.
I usually make up 1gal of idophor when I brew. The key is making sure everything is clean first. This allows the sanitizer to be most effective.