Long. Printers are pretty complicated machines, because they have to work with a natural product that shrinks, expands, folds, rolls itself up and sticks to other pieces of paper. I once heard a printer engineer explain that they use small puffs of air to lift the paper, but because there's also heat involved in the printing process that the paper sometimes rolls itself up or expands which causes jams etc. And I'm sure there's more going on.

Which isn't to say that HP aren't bastards.

not until the existence of 4D printer, that can print a 3D printer

You can already 3d print a plotter, which is close.

Likely not for a long time.

3D printers aren't a magic make anything tool. They are a versatile and useful addition to our toolboxes.

You could likely print a lot of the parts now. Unfortunately, it would be the "vitamins" that would catch you out. Circuit boards are possible, but difficult. Silicon chips are currently impossible. Print heads would be almost impossible to print too.

Instead, I would expect someone to come up with a more general "auto fabricator". A combination of tools combined with robotics, and a standard set of "vitamin" components. Such a system is perfectly feasible (though not that soon) and could go from raw materials to a functional 2D printer. It could also make a kitchen blender, a new lamp, or whatever you decided you wanted (within reason).

An interesting take is the book series of the bobbyiverse, starting with "We are legion, we are bob". They play around with the limits of 3D printing, and how to go beyond them.

Tl;Dr 3D printers are awesome, but not a "do everything" tool.

If you have enough money, you can technically do so right now. The only thing you wouldn't be able to print assuming money was no object and access to certain tools was readily available are the magnets for the motors. At least I don't think... Can you melt a magnet down and reshape it and it still is magnetic? 🤔

There are metal fab printers and the way circuits are mass produced is also, essentially, 3D printing. You couldn't do a print-in-place type print and make a completed printer. Unless you combined everything into a single unit. Which would be expensive as fuck.

It is possible right now. OP didn't say an electrical printer, it is 100% doable on old mechanical printing model.

Inkjet print heads are MEMS, so the answer would be until we can 3D print those.

You can start here: https://hackaday.io/project/176931-hp-printer-cartridge-control-module/details

HP printers are conceptually quite simple devices, the printer just moves the cartridge and the paper. The cartridge does all the actual printing. So you reverse engineer the pinout on the cartridge and you can make your 3d printer do normal printing. That's also how those little handheld cube printers work.

limx->0 1/x