Our Cleric lost 3 fingers due to a crit. He was on some feral world, and there was no one able to re-attach them. So, back in Scintilla, he asked if he could buy "replacement" fingers (not metal, flesh). But it seems this is not contemplated in the rules.

So I guessed that this is simply not the kind of medical knowledge they have in the year 40K. Then he asked for bionic fingers… also not in the book… then for a bionic hand… also not in the book. So he said, ok just give me a full bionic arm.

That was in the book, but I didn't find any rules for convalescence… do you just go there, and the day after come out with a bionic arm?

If someone could explain the whole procedure to me, I would be very grateful.

thanks in advance

I can't call to mind any examples in source books, but in the fluff (Black Library books) there are some examples. Generally the area has to be carefully prepared for some weeks in advance of the operation (for example by having a gel bag tied around the affected area to stimulate nerve growth or somesuch), which in-game could require a considerable period of downtime (or at least consideration as to how it could be accommodated whilst continuing one's duties).

Regarding the operation, I'd suggest taking real-world examples for convalescence time. You could considerably reduce the amount of time it takes to get used to the bionic and make it function as a flesh limb would, though (given, at present, it often takes people months to accustom themselves to a bionic hand and it would probably be at most a few days in Dark Heresy) as we should assume some progress has been made by the 40th millennium (despite 10,000 years of backsliding).

I'd suggest just letting your player have a bionic hand (unless they want the whole arm replaced, which I'm sure the Mechanicum would approve of). Note that the fitting of bionics would require both a chirurgeon (medicae checks) and a tech-priest (tech-use checks).

I believe Ciaphas Cain has some bionic fingers, so maybe even that could be accommodated, just reducing the cost of the bionic accordingly.

I hope this was at least somewhat helpful!

I would use a bionic hand, give negatives to actions involving fine dexterity for a poor quality one etc. Let it cost 1/3 of a full bionic arm. Attaching as stated above. Definitely going to take a few days to get completely used to and heal up but not months.