If a trust is created for a defined period of time it will usually include a vesting date, at which point all trust assets will be distributed pursuant to the trust deed, the discretion of the trustee/s or the court.
You no not need to include a set vesting date when establishing a trust. Trusts cannot exist forever. Most Australian States and Territories regard the common law Rule Against Perpetuities, which states that after 80 years a trust will vest and be wound up. In NSW this rule in contained in the Perpetuities Act 1984 s 7(1). The rule can differ on a state-by-state basis, to the extent that it does not exist in South Australia.