While these devices take a fair amount of energy to produce, I'd be careful about labelling their construction, and economy which surrounds them, unsustainable based on that - since it then opens up a minefield of questions about how you can use them for a set period of time to become "sustainable." The questions it opens up in this regard don't actually make an awful lot of sense.
When we're talking about permanent, or semi-permanent technological devices designed to manufacture energy, such as solar panels or wind farms, we can make this judgement call. We can calculate that the energy required to make a square metre of monocrystalline solar cell is
x, and that the average energy it produces over its lifetime in a UK climate properly positioned on a south facing rooftop is
y. It's then a relatively straightforward comparison.
But with devices that don't produce energy, only consume it, this is much less clear cut. What even counts as breaking even? Doing a certain amount of work, making a certain amount of calculations? Being in service for a certain amount of years? The degree to which the containing components can be recycled? Since they don't produce energy, no such device will ever break even in a simple, single unit sense of the word.
I believe this is the reason why there is no commonly accepted "minimum time" that you should own and use a device before disposing of it, and the challenges we face most in this regard are much more along the lines of how to properly recycle and manage this level of waste that we just didn't have 15 years ago, rather than how to work out if we've somehow "broken even" on the energy used to produce the device, and can now morally move on.