When you do your annual tax return you can include your RA contributions and are able to get money back. The tax deduction is limited to 27.5% or a maximum of R350 000 of your taxable income.
If you do not invest 27.5% of your taxable income per tax year, it does not roll over to the next tax-year. There are no rollovers allowable on contribution limits.
For example, if you earn R200 000 in a tax year, you can contribute R55 000 (R200 000 x 27.5%) and enjoy that as a tax deduction in that current tax year. If, however, you only contribute R10 000 in that same tax year, you will not be able to claim the underpayment (the R45 000 of the R55 000 you didn't use) in the next tax year.