I would start by looking at how many databases, schemas, tables and views are involved. Then look at how many individual Oracle functions need to be converted to plpgsql. You also need to investigate if there are any custom data types. I do not have the formula, but I am sure there is a general time factor involved in converting x databases, y schemas and z tables. You did not mention what hardware is involved, but I am also sure there is a time factor involved in copying / converting data from Oracle to PostgreSQL, and you need to consider how much data you need to move and the order, as there are probably foreign keys involved also.
You might want to do a small test to see how long it takes to dump 10k rows of data from 1 tOracle table and load to PostgreSQL. That will at least give you a general idea of how long it will take to move all data.