Hydrolock is when the water that comes into the piston cannot be compressed like air does.

As the piston tries to work its way to the top of the cylinder, it cannot go any further, and bam, bye-bye rod.

The answer to your question.....right away.

I knew a friend who did this with a stock Ford ZX2, about a month old. Don't ask me how, she must have been completely underwater....but her block was totally destroyed. Hard lesson to learn. OK, I'll say it......freakin' stuuuuuupppppid.