How long to flush?

What is considered an ample amount of time to flush a motor after use in salt water? Does the size and type of engine matter ie: outboard vs I/O, V8 vs V4...
Do any of you use the salt cleansing agents if the boat is used weekly?
I have a GM 5.7L and an OMC 800. I have been flushing for about 10 minutes which seems like an eternity.

Thanks in advance,

BassOMaticII