"Realistically, in all big investigations, longer than five years is bad," Michael Lauber told SRF radio in an interview broadcast on Saturday. "It always depends on how the parties to the investigations work with the attorney general."
Lauber said last month the probe, which began in March, had not yet reached the half-way mark.
FIFA's awarding of the 2018 and 2022 competitions to Russia and Qatar is one of the strands his office is investigating.
It has also opened a criminal investigation into FIFA President Sepp Blatter, a move that has led FIFA's ethics committee to suspend him.
Lauber told SRF that the level of FIFA's cooperation with his office had been good but could still be better.
He also said the OAG was currently not investigating the bidding for the 2006 World Cup.
The German news weekly Der Spiegel reported on Friday that German soccer officials had used a slush fund to win votes and land the hosting rights, something German Football Association (DFB) president Wolfgang Niersbach denied on Saturday.
(Reporting by Joshua Franklin and Ruben Sprich; Editing by Kevin Liffey)
