I think the thing that makes me feel OK about this move only 2 weeks before the season starts, is the way de Boer took over at Ajax. He was not hired in May or June and given the whole summer to prepare his team. Martin Jol was manager of Ajax at the start of the 2010-11 season. Jol went 9-5-3 in the clubs first 17 matches. He only won 1 of his final 5 league matches in charge, which lead to his firing. He also went 1-1-3 in the group stage of CL, losing to Real Madrid 4-0 at home! Ajax was in 5th place at the time of de Boer's takeover.
Frank was put in charge on December 6th and his first match was against Milan in the San Siro on December 8th. 2 days later! Not only did de Boer win the match against Milan 2-0, but he also won 13 of the final 17 matches in the league, defeating Twente on the final day to jump them in the standings and win the Eredivisie title. He did have the winter break, which lasts a month in the Eredivisie, after his first 2 matches in charge, but he has experience getting a team together in a short amount of time.
However, his second season in charge started off awful. If he had a similar start to that here, I could definitely see us firing him. At least if things were run like they used to be. I think Ajax fell all the way to 6th or 7th place that year by mid season. Not sure what ignited them, but they won the final 14 matches of the season, scoring 44 goals and allowing just 6!
In fact, de Boer showed a similar pattern in his first 3 seasons in charge. Ajax always started off slow when he was in charge and then finished strong each year.
2011-12
First Half: 9-6-2, 4th place (5-6-2 in first 13 matches)
Second Half: 14-1-2, 1st place
2012-13
First Half: 10-6-1, 3rd place (4-6-1 in first 11 matches)
Second Half: 12-4-1, 1st place
2013-14
First Half: 10-4-3, 2nd place (5-4-3 in first 12 matches)
Second Half: 10-7-0, 1st place
So let's not be in shock if the results do not happen right away. We gave Mancini 20+ months, hopefully we show the same patience with de Boer.