Other comments have led me to wonder, when you get certified are you paying for the class, or the certification? To me, I feel like a good instructor shouldn't have a problem with giving some students some extra help, rather than failing them and making them take the whole course again.
You are paying for the class and instruction and the time it takes for an instructor to teach you. You are also getting pool time, manuals etc as well. If you took a night class at your college to learn another language and you fail to learn it does that mean you get your money back or that you should still be given a diploma/certification? No! Scuba is no different, although unfortunately some places are only in it for the money therefore with minimal skill and sometimes no skill you end up with a card because it's the easy way out for the dive shop. You get the card and they see the back of you without any additional time or work involved.
I think a good instructor (one that truly cares and is committed) should give you some help and perhaps at a minimal fee if any (that's their discretion). Also I don't see how they should outright fail you and expect the full fee again for the course. You should simply pay your portion of the pool rental and the instructors time for additional time needed to acquire the necessary skills. You may even be allowed to sit in on the next class and just watch the DVD again, read the book again and listen to instruction without paying any extra (provided there's room). If an instructor is fully committed then you should have nothing to worry about unless all avenues have been exhausted in which case diving probably isn't for you.
No idea on the time limit. I would guess you have a year to complete everything but I don't actually know that answer.