There is no time limit for being on short-time, although if it becomes clear that it is not temporary then you can argue that a redundancy situation arises (although there is no guideline as to what it is considered temporary.
You can of course opt to apply for voluntary redundancy if your short-time has lasted for 4 weeks or more, provided that your employer cannot guarantee you 13 weeks work at full hours/wages.
If you opt to continue in the employment rather than seeking redundancy, then make sure that you are regularly writing to the employer to say that this is a temporary situation, and that you have not accepted the short-time wages, otherwise if it does come to redundancy any calculation could be based on your reduced salary!!!!