Hi,
Sorry but it is difficult (if not impossible) to give estimates on this kind of questions. But is is well-known that the dbimport tool is not the best scalable tool there is. Normally I would remove the indexes etc. from the import file and run them seperately afterwards using PDQ.
I think it is wise to tune your onconfig for this specific task. I can tell you it will take quite some time.
Again sorry for the non-complete answer!!
Greetz,
Rob