Until some earth destroying, population decimating event takes place. I mean a market crash is inevitable, it's kind of baked into Capitalism and is an essential part of the system. If you are talking about collapse as in full collapse, then it probably won't happen within our lifetimes. If you are talking about a market crash, then probably within the next few years. Much more likely to be after 2024 since this is the point where a lot of very large economies (US, UK, India, South Korea) have their elections, and governments (especially so in the US and UK) are likely to want to postpone a market crash until after the completion of the next elections.
A stock market crash or decline in economy isn't really caused by anything more than people, collectively, thinking the economy is going to crash, like a self fulfilling prophecy. However a full financial market collapse would, in my opinion, only come after a huge world changing event, that most likely has to happen all at once (not climate change related disasters since the events won't instantly happen all at once). It would have to be something along the lines of asteroid/comet collison, supervolcano eruption or nuclear winter.