The migration to the new platform will probably take you at least several weeks and up to a couple of months, especially if new hardware needs to be ordered, installed, and configured. Adapting interfaces and output management to the new platform should also not be underestimated. In addition, in-house developed ABAP reports which use database dependent functionality need to be adapted to the new RDBMS.
The time needed for the upgrade to ECC depends on the number of SAP modules in use and the number of modifications to the SAP standard and the in-house developments.
This was first published in May 2007