After being unveiled as United boss in July, Van Gaal claimed that it would take three months in order to get the club back on track after a disastrous 2013-14 campaign under David Moyes.
However, the Red Devils have already dropped 10 points this season, and the Dutchman admits that it will take a little longer to get the Premier League giants to "rise" again.
"I have said it was very stupid to say three months because then you [media] are only talking about three months," he told reporters.
"It is always the performance that counts, but the performance always goes up and down. Then we rise. When that date is, or what that match is, depends."
Nevertheless, United go into tonight's clash away to West Bromwich Albion on a high after beating both Everton and West Ham United prior to the international break.