Assuming healthy plants to begin with 5-6 days is safe for all but the pickiest plants. When plants are shipped from overseas, they aften undergo more than a week without light. I have done as much as 5 day blackouts, and the only plant I've seen react adversely is my anachris, but even it didn't die off, only looked a little yellow and stretched when the lights came back on.
Some algaes will survive blackouts as said, green water, BGA, and most green algea I've had will die quickly, BBA seems to be the hard one to kill, but then it also seems to be the slow grower and easy to remove algea in my tanks when I've had it.
Dave