Is there a "maximum time limit" for nonconformity to fix, and if so, what would this be?
What is considered "undue delay"?
When you say "to fix NC" you are referring to corrections, instead of CAs, right?
Look at your contract for these definitions. All contracts have a schedule (even if not explicit, via the applicable law). You are required to deliver product that meets the specs by a given date. If you have a NC, you usually have to correct it before the agreed delivery due date. That's your "maximum time limit". If you can't, then you'd likely have an "undue delay" unless your contract has relief provisions (such as force majeure). Some complex contracts may have provisions for CA timing too -- check those as well.
If you are in the rare situation that your contractual schedule is not binding because you have stricter internal programs and associated procedures, then look at those programs and procedures for the definitions.
Finally, if none of that is binding, look at the standard under which you are registered. ISO 9001 does not dictate a correction schedule, but perhaps others do.