For how long must I be married before my ex-spouse can claim from my retirement fund?

It can be any period of time; there is no minimum requirement.

In terms of the Divorce Act, upon divorce, each party’s pension interest (in a retirement fund) is deemed to be part of his/her assets which are taken into account in determining the patrimonial benefits to which each party may be entitled. The only exception (where a party cannot claim against the other’s pension interest) is when you are married out of community of property and the marriage was entered into on or after 1 November 1984 in terms of an ante-nuptial contract by which community of property, community of profit and loss and the accrual system are excluded.