Yes min. 5 yrs unless the gov't has changed the rules. If you renew the 3rd contract, you can still avoid the long service pmt by terminating the contract before 5 yrs is up and get a new maid. HOWEVER, you will have to give very good reasons or the maid can sue you for unreasonable dismissal. There have been cases that the judge consider the employer dismissing the maid unreasonably just to avoid the severance/long service pmt and ruled in favour of the maid.
Martina,
Yes you need to hire a new maid and give good reasons for not renewing the contract with the existing one.
I am not supportive of dismissing maids just to avoid the payments. My own opinion is that if I have a maid that is good enough for me to keep her for several years, and I let her go not because she's bad, but because kids are already grown up, or we are relocating to another country, etc., I would give her the payments that she deserves.