Between 4 and 24 weeks. If you want to take the program for longer than 12 weeks, you’ll need a working holiday visa (subclass 417)  or a work and holiday visa (subclass 462). 

If you have previous experience with horse or farming, or you have lived on a farm before then you can apply to stay for up to 24 weeks.

the maximum duration for someone with no experience is 12 weeks. You may be able to extend once you arrive and if everything is going well. It will be up to your farm family if you can stay longer.