The first 90 days in the new role can determine the success or failure of the new IT manager. Most colleagues expect a new leader to bring change. This gives you an unrepeatable opportunity to take the quick steps needed to improve IT.
You also need to remember that in the first 90 days you will be making an indelible impression on most people. They will be making up their minds about your contribution to the company and how you solve everyday problems. If you prove yourself, you will gain the trust of colleagues and top management.