When the price suits!

If you can easily / cheaply position to AMS it’s about �400 cheaper

First Class Reward seats not so rare on LHR-JNB so maybe check before booking as you could pay AMS-JNB in J and then upgrade using Avios to F, good ish use for some of those 150,000

If �1.5k suits your budget then book and don’t look back.

If looking for an Avios redemption then the 2 guaranteed J seats will have long gone. If BA release more it could be at any point though often quite late in the day which reduces your options for cash fares if it doesn't materialise.

If considering cash fares then my ballpark figure for a "good" price from AMS-JNB is �1800. If that's available when I first look then I'll take it, if not & I've got more than 3-4 months & a bit of flexibility with dates then I set up a google flights alert & hold fire. I'd be wary letting it get below 10 weeks out though. Obviously if you are less risk averse or need specific flights then may want to book earlier.

Thinking of F upgrades which are generally a good use of Avios then unfortunately BA recently had a fairly popular F sale to CPT for Jan-April next year which has probably filled most of their First seats to SA they would sell cheaply or release for redemptions so I wouldn't hold my breath waiting for an upgrade opportunity but you never know!