You can fight a foreclosure within the time period set by law (usually 20 days after you were personally served with the foreclosure Complaint, 30 days if you were served by mail or other means). Once that time has passed, you are legally in default and your time to answer has expired.

This is not, however, the end of the world. If your time to answer has expired, you will need to get the court's permission to file and serve a late answer. Many courts will allow you to do so if you have a good defense in the case, as well as a good reason why you didn't file an answer in a timely manner. It's also smart to call the bank's lawyer to get their consent, in which case you wouldn't need to get the court's permission at all - but if they don't consent, letting the court know in your request will show that you've done your best to solve the problem on your own.

If you are served with a foreclosure and file for bankruptcy during the time to answer, then that time clock stops running while you're in bankruptcy. So if you're served with the Complaint and then file for bankruptcy on Day Ten, you will still have at least ten days to file an Answer once the bankruptcy ends (or if the bank gets relief from the automatic stay).