The massive amount of customizing is normally there because what many companies believe is that their own processes is what make them successful. That's why the software has to be bent to fit the processes.
In my life as an SAP consultant I have met only 1 customer who bent their business processes to fit SAP 100%. They need only 1 FT SAP service person for the whole country (it was Belgium).
With many consultancies offering packages that can easily be imported to kick-start the SAP prototype, the implementation time has been reduced to minimum nowadays.
I am working 60-70% as a technical consultant. What I noticed is that a lot of time is spent in the recording of the business process in a coherent form, sometimes longer than the calendar days needed to complete the implementation. That is actually the bottleneck in any ERP implementation I assume.
Another issue is that often is believed that a project manager who is an expert of project management can handle an SAP project without prior SAP knowledge. That's a mistake and produces frictions inside the teams. This type of PM's are normally those that go over budget :). The only case where this can work, is if that person is the topmost project manager who has part project manager reporting to him/her who have SAP knowledge.