This is from bwjokes.

A young American tourist goes on a guided tour of a creepy old castle. At the end of the tour the guide asks her how she enjoyed it. She admits to being a bit worried about seeing a ghost in some of the dark cobwebby rooms and passages.

“Don’t worry” says the guide, “I’ve never seen a ghost all the time I’ve been here.”

“How long is that?” asks the girl.

“About three hundred years.”