Real time to get Windows 7 functioning as XP install
Real answer? 0 mins because I installed 7 to function as 7.
(well, somebody had to have a pedantic response to the title
Realistically, to install 7 and have everything functioning similar to their XP installation is going to take varying amounts of time due to to an individuals preferences, hardware, applications and adaptability.
For some, this process will be quick. For others - the OP for example - a significant amount of time will need to be invested.
There is no single quantifiable amount of 'time needed'.
The reason it is not akin to a service pack update is because it is not a service pack. It is an entirely new Operating System.