What is the longest time period for a travel visa to China for a South African?

Tish, usually, one may be given a 30-day tourist L type to PRC.
If you don’t have residence permit in HK, it’s difficult for you to apply from there.
As you said, you have student X type in PRC, before it expires, you can go to the exit and entry administration of the local PSB to apply for a tourist L types. In most cases, you may be given 30 days to handle relevant procedures in PRC.

The required documents include your passport and copy of its data page, the application form attached with a passport-style photo as well as round trip air tickets. You should apply for L type 7 days earlier before your X type expires.

It usually takes 7 working days to process your application.

Thank you for your assistance Karl, very helpful and much appreciated 😀