First I wonder why there should be a “downtime” at all? Normally loading data does not require to be executed in any kind of standalone mode.
Second, the time for loading the data depends on the method choosen to load the data. When inserting the records directly into the database tables with open sql will be faster than loading the data via bapi or direct input programs.
Third, the complexity of the master data is determining in some way the time requirement for loading. When only the minimal data content is loaded, the load will be faster than with fully filled data structures.
Intuitively I would roughly estimate a load time between 4 and 8 hours as an average. I’m eager for other experts’ opinions.