Sorry to be a responder with such little experience, (cleaned maybe 30 pieces) but I read on this forum that you can leave it in forever and it won't do anything but eat the gunk and rust. I've left things in for ten days in one case to clean a piece. Someone said that they'd had a piece in soup for months, if I recall correctly. I'd worry more about if the rust has eaten through the manifold already, in which case you're out of luck.
When they said left it in the "soup for months", they were talking about a lye solution, not electrolysis. As for hurting it in electrolysis, the difference between leaving it on longer is just in your utility bill, I'd think. I've left in on for two days, just to be doin, and it never hurt anything. The water got a little hot. I would not recommend leaving it on and just leaving though. That is until you set it up and get comfortable with it by cleaning several pieces.
There are a lot of variables in using electrolysis, none of them bad but just variables. Like, the piece you are cleaning will seldom be the same size, no amounts of gunk and rust are the same, the anode (the piece you use in with your iron) will gunk up and that affects cleaning time as well. So, there are a lot of variables and those variables change. So, until you know what you're doing real good, I'd keep my eye on it pretty close.
Like I said, I've left it on for a couple days, but thats me and my conditions and what I'm cleaning, and not you.
