Forever
"And, this one. This is the one you have been telling me about?"
"... ye ... yeah, yes! That's the one."
"How long has it been running for now?"
"We ... um ... we don't know."
"What? What do you mean? Is there a log of when it started"
"We've looked. On this machine, it goes back to the date the process started. And, the ... uh ... the one before that goes back to the date its process started. So on and so forth."
"Huh? What is it doing then?"
"We, ... uh ... we aren't quite sure. Maybe it's generating something. That's what these things were originally used for. You know, back in the day."
"Do you think its doing that now? Generating something?"
"Well ... ahem maybe? It's difficult to verify. We tried unplugging it some time ago ... and ... uh ... it just starts again on another machine."
"Did you completely isolate it before unplugging it?"
"Yes, and it predicted that and started again on another machine. We ahem, ... we tried numerous times."
"sigh How long do you think it will continue?"
"... I, ... uh ... my guess? Forever."