Forever. Remember the machine ID is a unique install even on the same hardware. If the machine has been sold and reformatted then it's ID will never pop up again, but any work done by that machine ID is tied to it forever. If work cannot be audited back through it's entire life then it cannot be trusted.
If you just worry about the active machine list you don't have to bother yourself with this stuff. If the machine ID isn't active, ignore it. Problem solved.

Your example is faulty since the registration plate isn't the unique ID in that case either. Just like the device ID here, you never actually see the unique key for those records and probably never will.

Let's extend that car registration analogy based on how it actually works.

The rego plate on a car can be changed for a personalised plate any time you want to cough up the money. This doesn't make any difference to the recorded history of that vehicle. Those same plates can also be taken from one car to another without merging the history of the two vehicles. That's because the rego plates are just a "name" tag associated with the car and not the actual record key. In the same way the device/host name of any device on your personal list can be changed any time you change the name of that computer. The device ID, however, is the record key and that never changes. It may be handy to be able to change the device NAME from an interface on the website, but the device ID is for WCG internal accounting systems and, quite frankly, none of our business. More than one person has confused the device name and device ID. They are not the same.

Bubbadus, if you use the normal options on the My Devices page your old retired devices won't be showing up anyway. Messing around with WCG's records will not gain you anything new. As for the stats it won't make any difference since the average is from the time you started and will always include everything.

It's the record ID, something that's created when the vehicle is first registered. It's only used internally as a fixed reference point. You could use the chassis number in most instances except that it's usually attached to the firewall and if you get a body-shop to replace that (big job I know) after a traffic incident then that might get complicated as well. These numbers don't have to relate to anything physical, it's just a fixed, searchable point that holds the records together.

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No, it's totally accurate and your example does nothing to illustrate otherwise. The device ID refers to a logical device, not a piece of physical hardware. It is possible to move that logical device around by copying all the configuration files from one installation to another even though that was not the intention of the software designers. If they'd known this was going to cause such tantrums amongst non IT users they might well have tried to include something from the CPUid and Motherboard hardware as well as the Windows product validation code (it's in the registry hive) and a random additional factor (with some other variation for Linux, Mac, Solaris, FreeBSD and every other OS that has a BOINC client), but they thought we'd be adult about it and didn't see the point in wasting coding time. Copying the data folder is a work-around used by people who get their knickers in a twist over a number they don't understand, nothing more. If you don't think that device ID is unique just try and copy an install from one machine to another and run them both at the same time. Since a given logical device is linked to one and ONLY one Device ID code, running two devices at the same time with the same Device ID will result in one if not BOTH of them having all the work in their caches aborted and a new Device ID created since that initial ID has been made suspect by the duplication.

So-to the car registration plate is NOT the unique key for a vehicle, as I already said. Any field that can be reassigned to other records is by definition NOT UNIQUE. In this case it is ONLY useable at a given point in time since a given registration plate can be reassigned to another vehicle without altering the other associated records for EITHER vehicle. The primary and unique key is ALWAYS unique and used to index those records for as long as they exist.

The end result here is simple. The system is the way it is FOR GOOD REASON that some people plainly don't understand. If the work is what matters to you, GREAT! This is all utterly irrelevant so let's get crunching! If you're more interested in having a hissy fit because you can't alter project records to suit your own personal whim, feel free to find another project. None of them will let you mess with their auditing records either, but you'll obviously have to find that out for yourself and tantrums about it will do you no more good their than here.