The performance analyst worked with Guardiola at City, along with Bayern Munich and Barcelona.
Planchart told Sport: "It's a personal decision he'll have to make. I think a project should last five or six years, no more.
"But not for him, for everyone. Afterward, you have to regenerate. As a friend, I would tell him to look for a new project because he still has a long way to go.
"This is why he's been at City for so many years: they've treated us like family, they've let us work as if we were at home. He didn't feel that way at Barca or Bayern.
"He's a football fanatic. His life is on the green, on the grass. He's a genius, a creator. His greatest strength is how he invents football. The difficult thing in this life is creating; the rest of us are copycats. He's number one at this."
On City's struggles last season, Planchart also said: "Sometimes you lack energy, and when you get into a bad dynamic, it's hard. In football, you always have to be at 100 percent. We had injuries, people were at the end of their careers.
"There was a drop in performance, even among the staff. The lack of energy forced us to fight just to get into the Champions League, and it was an achievement. We also lost the cup final; it was punishment for a poor season."