The time will depend on more than the number of bytes. The number of indexed fields, the number of indexed calculation fields will be a factor as well as other issues that are more a case of complexity than file size.
That said, it will take a while and yes you'll use FileMaker PRo 10 or FileMaker Advanced 12 to convert the file. I'd set this up at quitting time, kick it off and then check back in the morning to see if it converted successfully. You may need to convert it twice--once from a back up copy so you can test the converted copy to see if it works OK and then a second time with all the latest dat edits in order to bring it back up as a Filemaker 12 file.
I was hoping someone had some practical experience with FMP12 and actual timing data. Maybe that will come in the future once people start buying it.
Their timing data would not be a good prediction for you as their file will not match yours. The number of records and the number of bytes to the file are not the only factor in determining the time required. I suggest you experiment on a backup copy of your file and see how it goes.