Some reports have suggested that Ferguson, who took charge at Old Trafford in 1986, would retire at the end of the current campaign.
However, Charlton has claimed that the 71-year-old could keep the job for another decade.
"It could be next week [when he retires], it could be 10 years, it doesn't matter. We will wait for him. We can't tell him what to do, he is fantastic," Charlton told talkSPORT. "He loves the game and every times he thinks about retiring he has to look carefully at what decision he is making because it is a tough thing for him to do.
"He is so positive and knowledgeable about the game that he won't make any mistakes. If he retires, it will be his [decision]."
United have won numerous trophies during Ferguson's time at the helm, including 12 Premier League titles, five FA Cups and two Champions League.