Depends on stock, and on needed accuracy.
And machine/vice.
Its different for 90 degrees,
90.0 degrees,
and 90.000 degrees.
Anyone can do any of the three, with modest equipment.
Imo, the third might take 3-4 weeks, first time.
(Lap (CI or brass/tin, etc) and check via 2-3 identical pieces or lab squares or optical flats, etc.)
And it should have qualifiers for the last, imho.
Surface wont be flat to 90.000 degrees level, RMS values likely high, edges will be rounded, etc.
Imo, ime, engineering is about finding compromises suitable for the task at hand.
Cost or time tends to go up 5-10x for every digit.
If you have time, a few hundred 100€ can get you lab quality results and tools, very slowly.
(Example: Amateur telescopes).
Or, a 100k tool can do the same in 5 minutes to 1/2 hour.
(Example: Tool scanner with optical measurement, 0.5 micron accuracy, non contact.
I have seen it done it, at ISCAR over here. 2 minutes).
Imo, typical cheap milling or grinding might do 2 decimals, or 90.00 degrees, some uncertainity on last digit (ie +/- 1 unit).
Cheap(ish) one or 2-sided lapping machines will be much better, to 1 micron accuracy in flatness and angle, in ten-twenty minutes or so.
Dont have the lapping machine, but looked at them and talked to the manufacturers, over at EMO trade fair in germany.
Saw it done, very simple process.
Vids on YT.
