Hmm..good question.
As for the flush...what type of system are you using? If it's mediumless hydro and your plants aren't loaded right up with nutes get some flushing stuff. 3 days might be fine.
If you're in soil ..well..unless you've got a real mild nute in there you COULD be in trouble.
BUT..some people don't flush at all and say they dont notice a difference.