The quadruple world champion is remaining an ambassador for the French carmaker, after Renault bought back its old works team that in recent years has been known as Lotus.
“It will not be possible already in 2016 to compete against the best,” Prost, whose works Renault E-Dams team won the inaugural Formula E series, told Der Spiegel.
“It makes no sense to lie to people — it will be a very difficult year, especially at the beginning. It will be necessary to be patient,” he added.
Prost said Renault’s sights are set much further into the future.
“This is obviously a long-term project, which is normally nine years but it could be more,” he is quoted by the Spanish news agency EFE.
“They do not need to win the first race in Australia. The expectation is to be one of the top three teams quickly, but that means perhaps two or three years and maybe four.”
Prost said Mercedes is once again the favourite for 2016.
“Of course, but the championship may be somewhat more open with Ferrari,” he said.
“If Ferrari was able to do it I think it would be good for formula one,” Prost is quoted by Italy’s La Gazzetta dello Sport in Paris.
New Renault driver Jolyon Palmer, however, thinks Mercedes is the clear favourite.
“They have been the dominant team for two years now and the rules are not changing,” he is quoted by the Spanish sports daily Marca.
“They have the best engine and a very good car, so I think yes, they are the favourites again.”