It mostly depends on conditions – Dry and Molten Speed Wax will last up to 800km (but recommended to do earlier to be safe), wet conditions the chain should be wiped dry and put back into wax asap before any oxidisation occurs – just like after a wet ride with oil you would wipe chain and immediately re oil. (*note, you may be running a wet ride lubricant which does not need immediate attention after adverse conditions rides. But they are typically very high friction in the real world. Sure, the lubricant may stick to your chain, but so does most of the road grime spraying off your front tire which is now packed into all the hard working links).
I personally like to re-wax more often. Being a solid lubricant, I find after about 250 to 300km the chain will start to feel dry. It is still most definitely lubricated everywhere it needs to be for much longer, it’s just that without all the grinding paste filler that you have with oil, it will feel dry. I find the first approx. 250km feels like you are riding on pure silk, so I re-wax around this mark (which is comparable frequency to generally better performing lighter oils).
Accordingly – If waxing at long intervals, Molten Speed Wax recommends changing wax for training chains after 8 waxes as a certain amount of contaminants will be introduced into the wax.
I wax at short intervals and personally go around 20 to 30 waxes and am seeing outstanding longevity results.