Dhoni, 26, and Sachin Tendulkar are in the running to become the captain of India's Test side when selectors meet later this week to name Rahul Dravid's successor.
Dravid relinquished the post in September saying the captaincy brought him too much stress and that he wanted to focus on his batting.
"He (Dhoni) is doing his apprenticeship in the limited-over forms of the game. It won't be long before I think he is ready for (the captaincy of) all three," Chappell told Times Now news channel on Tuesday.
"Probably another 12 months or more I think before he is perhaps ready for all three of them."
Dhoni, who led a young team to success in the Twenty20 World Cup in South Africa, was appointed captain of the one-day team for 12 matches concluding with the ongoing five-match series against Pakistan.
India play three Tests against Pakistan at home starting later this month before touring Australia for a much-anticipated four-Test series.
"A tour of Australia behind him will just finish him off nicely as a cricketer and as a potential leader," said Chappell, who quit as India coach after their first-round exit in the one-day World Cup in the Caribbean in March.
Dhoni's leadership ability was lauded during the recent home series against Australia which the visitors won 4-2.
The wicketkeeper-batsman guided the side to a five-wicket victory over Pakistan in the first one-day international on Monday.
However, Tendulkar is seen as a strong contender for a third term as Test captain, which would mean an unprecedented two-captain policy in Indian cricket --- one for Tests and the other for the shorter formats.
Tendukar led India twice earlier without much success, winning just four of 25 Tests before stepping down to concentrate on his batting.