It doesn't matter how many rows you knit, you just keep knitting until your scarf measures 70 inches (or the length that you want it to be). Every knitter is different, so it will take different people a different number of rows to achieve the same 70 inch scarf. This is why a lot of patterns give their directions in terms of inches rather than rows.
As for casting of in rib, you just keep working in your rib pattern as you bind off ... so, during your bind off row you work each stitch as you would according to your pattern - the only difference is that after you've worked the stitch you bind it off rather than leaving it on the needle.
There may be a video here that shows binding off in a rib pattern, I'm not sure.
If not, perhaps someone else will be able to describe the bind off better for you ... I don't feel like I'm putting it into words very well!