ExpertFlyer says that BA-BA domestic-international at JNB is 90 minutes. The only exception is CPT-JNB-HRE which is listed as 85 (presumably because of a specific scheduling issue):-
Code:
Minimum Connect Times:
Connecting at JNB
Incoming airline BA
Outgoing airline BA
Flight type Domestic to International
STANDARD.D/D...D/I...I/D...I/I.
ONLINE 1.00 1.30 1.30 1.00
OFFLINE 1.00 1.30 1.30 1.00
** OR * ARE ALL
BA-BA DI 1.25 CPT - HRE
**-BA DI 1.30