They take their interest based on the outstanding balance. Should you need money, you can then again withdraw whatever surplus is left up to the total loan amount
If your revolving limit is R50k
You use R40k
At the end of the month they charge you R1500 (random example) in interest.
You now have R8.5k available
You pay the R51500 interest and choose to add another R1500
You now have R11.5k available
The cycle is allowed to continue until infinity. It works almost exactly like a credit card, but you can't exactly go to a store and purchase with a loan account
A term loan is as an example 24 months. If that outstanding balance is not paid by the 24th month, the bank can take legal action.