How often does Blacksmith's stock reset?

Their stock reset every 3 days. Unsure about their coins.

The vendors' coins restock every 5-7 days. However, they only restock a partial amount of their starting coin.

From multiple sources, it looks like the merchants' restock rate for gold is 5 days.

See here:

User 1: Never checked when, but heard every 5 days.

User 2: Yea every 5 days....

here:

thay restock every 5 in game days

here:

User 1: So vendors restock money after 7 days? I didn't know that. Back to cow farming

User 2: 5 days actually, dont waste that extra time

and here:

If I recall correctly, you need to meditate for 5 days for a merchant to recover some of their gold.

NOTE: It looks like they will only restock some of their gold, however, not all of it.

Okay, I may be 5 years too late to this, but here's what I observed after searching for the same question, and doing what the first answer suggested.

The seller I'm talking about here is the blacksmith in the fish market in Novigrad (the map might not show this one to you, but that's a different story). I'm level 18 and this guy has 1215 coins and the restocking happens as follows:

1. You over buy and he's left with few tens of coins (say, 11) and he can't buy any more swords from you. In this case if you pass 5 days in game by meditating or otherwise, his coins will be replenished to 280 + whatever he had left with him previously (meaning 291, in this example).

2. If you buy till the point that he has zero coins left, he will get a) 935 coins in a few minutes, say by the time you run back to your stash and come with a new set of swords and b) 1215 coins, which is his original number, in 5 days.

TL;DR: if you want the merchant's funds to replenish quickly, make them absolutely poor buy selling stuff till they have exactly zero coins. Tip: Junk and crafting items like wire, etc. could be useful in this regard.

Edit: I made a little video explaining this.

Hope this helps!