There are lot of variables to answer that question. I do not think there are any straight answer to that.
Depends on complexity of the project, Procurement and Inventory does not mean only those 2 modules. Both of them are connected to address book and finance.
What kind of Data we have, are there any custom process? if it is only standard process with minimal data, How many resources.. etc...
To my experience i have not seen any implementation project happened less than 4-6 months.