How to do data migration between different schemas?
Did I understand this correctly that your customer has MSSQL Server?

Source Database MSSQL or other

You have two options:


Script your own replication
Use Oracle Golden Gate to stream the data to your Oracle Database. Golden Gate supports "zero downtime" migration.


Source Database Oracle

Additionally to the options mentioned in "Source Database MSSQL or other" you can do:


Use Streams, Basic or Advanced Replication Features (requires Enterprise Edition, Basic Replication is included in SE, SE1 and SE2)
Export/Import DataPump (this does not support continuous replication)
Import DataPump via database link
Transportable Tablespace (requires Enterprise Edition)




There are several other posibilites but I need more details to recommend you one method:


How big is the customer's database? Please use the view dba_tables to get this information. The size of the datafiles does not reflect the actual size of a database.
How fast can you copy 1GB from the customer's side to your side?
Do you want to convert and cleanup the data while transfering it to your side?
Which method did you use when it took 6 days?