When 64 bit (or other) systems become more cost effective (including vs already owned systems).
In the realm of traditional Windows laptops and desktops, 32 bit systems are already largely obsolete. If you go to buy a new computer in this category, you’ll almost certainly be getting a 64 bit processor. Even Intel’s Core M processors are 64 bit. The remaining 32 bit systems that are in operation are mostly pretty old now, and dying off, so in the traditional PC realm, you’re looking at a timeframe of a few years before essentially all operational PCs are 64 bit, at least as far as the processor goes.
In the smartphone/tablet world, 32bit has held out longer.
Apple introduced 64 bit on the iPhone 5S. There are still folks using older iPhones than that (a friend of mine just recently replaced her iPhone 4), so we’re probably looking at a couple years until most of the 32bit iPhones are out of regular use.
The iPad Air was the first with a 64 bit processor. From what I’ve seen, iPads aren’t replaced as often, and there are plenty of iPad 2’s (maybe even original iPads?) still being used regularly, so I would guess we’re looking at more like 3–5 years until the 32 bit iPads are largely no longer used.
Android devices are still a mix of 32 and 64 bit offerings. I would guess they’ll all go 64 bit within 2–3 years, so you’re probably looking at something like 6+ years until the last of the 32 bit devices are retired.
Then there are other devices, such as IoT (Internet of Things) devices. It’s very hard to tell when those will be largely 64 bit, as they tend to care more about low power usage and low cost. Your guess is as good as mine when they’ll all be on 64 bit.
Even nowadays 8-bit and 16-bit microcontrollers (and even “weird ones” like 18 or 24 bits) are still extremely common. It’s just that people don’t notice the existence of them. For example inside your washing machines, remote controls, air conditioners, cars… there are tons of microcontrollers inside. Those embedded systems doesn’t need to be powerful, they just have to be very reliable and power effective through the years.