Common practice is Acid then coffee but there in no law that says you have to follow common practice. Try it and see what happens

There are folks who do not have access to Ferric and only use coffee. I know a couple of them in Europe who do this.

The truth is, you can use any acid for the etch. The length of time required is indirectly proportional to the strength of the acid used.

Weaker acids take longer than stronger ones. I typically use a couple of short etches (5-8 minutes) in Ferric, followed by longer coffee etches (2-3 hours).

I have tried the coffee only etch and used a overnight etch with good results.

Let's not confuse etching with simply blackening. Probably 99 of 100 people who are using coffee are only using it for the contrast.

I have no idea how long the minimal acid in coffee would need to work for actual etching.

Armchair chemist here but if I recall correctly, ferric has a pH of ~2 (more or less based on your dilution mix) and coffee is around 5 or 6 making it a lot less acidic than ferric (neutral drinking water has a pH of 7). The etching time is directly related to acidity, in exponential terms, so it'll take A LOT longer in coffee for an etch than in ferric.

Like Karl said, it's more "blackening" than "etching". Correct me if I'm wrong, but I thought that a coffee blackening will eventually fade where an etch will not. With an etch, you are removing material and with a blackening you are adding a finish to the material.

Yep. As I said above, The length of time required is indirectly proportional to the strength of the acid used. Weaker acids take longer than stronger ones.

A purely coffee etch can take 8-10 hours to see any significant etching. An Italian maker I have spoken with has left a blade in cold instant coffee for 3 days to see any dramatic results.

I forgot i posted this I decided i like to boil in coffee after etching in ferric. how long I do not know yet as I have been working on my had polishing since I last posted this and am low on non slavaged steel.