Piracy: ꜱᴀɪʟ ᴛʜᴇ ʜɪɢʜ ꜱᴇᴀꜱ

What do you mean by "last"? I know it's a common term, but when you dig deeper, you'll see why it doesn't really make sense. For this discussion, I'm assuming you mean "How long until I need to buy a newer model?"

First, consider the reasons you might have for buying a newer model. The first is hardware failure. Second is obsolescence - the device cannot keep up with newer needs, such as speed, capacity, or interface. The third is insecurity/unsupported from the vendor.

The last one is easy enough to check from a vendor's product lifecycle page. I'll assume this isn't what you're concerned about. Up next is obsolescence. Obviously it meets your needs today, but only you can predict your future needs. Maybe it's fine for a single 1080p* stream today, and that's all you use it for. It will continue to serve that purpose forever. But if your household grows and suddenly you need 3x 4k streams, it might not keep up. Or maybe you'll only need that single 1080p stream for the next 20 years. Maybe you'll hit drive capacity limits, or maybe you won't. We can't answer any of that for you.

That leaves hardware failure. But electronics don't wear out (mechanical drives do, to an extent, but you asked about the NAS). They don't really have an expected life span in the same way as a car battery or an appliance. Instead, they have a failure rate. XX% fail in a given time frame. Even if we assume a bathtub curve (which is a very bold assumption), the point where failures climb is going to be very unclear. The odds are actually very good that it will keep working well beyond that.

Also of note, very few electronics fail before they are obsolete.

*Technically it's about bitrate, but let's just ignore that detail for simplicity. We'll assume that 4k uses 4x as much space as 1080p

TL;DR: It could fail at any moment from the day it was manufactured, or it could outlast all of us. Prepare for that scenario with a decent backup strategy, but don't actually replace it until needed.