Albion's interim boss believes all coaches have a shelf life, apart from those at the very top.
And he says it becomes more and more difficult to lead teams the longer you are in charge of them.
Pulis was sacked six weeks shy of his third-year anniversary at the Baggies and Megson told the Welshman about his theory, passed down to him from World Cup winner Jack Charlton, in the aftermath of the defeat to Huddersfield.
"I was talking to Tony the week before last," started Megson. "I was a player with Jack Charlton, I got on great with him.
"He said to me at Newcastle, if ever you go into management it's three years. The first year, you say jump, everybody jumps. The second year you say jump, they say how high? The third year you say jump, they say we've been doing that for two years.
"It becomes the norm and you become the victim of your own success and it becomes more and more difficult. Once you've been at any football club for a while it does get harder and harder.
"I think Alex Ferguson said you need to keep refreshing and changing the players. People like (Arsene) Wenger and Ferguson deserve enormous amounts of credit because staying at football clubs for that long is unbelievable."
Before Pulis was sacked, only Wenger, Mark Hughes, and Mauricio Pochettino had been in charge of one club in the Premier League for longer.