It depends on the paper. If it's FB, then it doesn't hurt too awfully much to leave it in the wash for that long. RC paper is another story. A quick 4-5 minute wash is all that's necessary, any longer and you could delaminate the paper and ruin the print. Ipersonally am not too keen on leaving any prints in the wash for over an hour or so, it sucks when you ruin an entire evenings work by leaving prints in water for so long.
Ah yes, very true! I would expect that an RC print might be in two pieces if you left it in water overnight! I hope the OP was talking fibre prints (I was)