Much depends on the type of sink....

But why are you going to hang a pendant over a sink anyway unless it is a small sink used for rinsing fruits and veggies on a peninsular or island?

Anyway, you'll have to consider the height of the person(s) who are going to be at the sink and try to balance the overall look (height of the pendant above the sink or the top of the faucet), against what the sink is being used for.

Personally I would start on the higher side (40 inches) if the sink is large and deep and you will be rinsing/washing large platters and filling large pots with water. Just the thought of taking large dishes and pots out of a sink and having to be careful about a pendant light will drive you crazy.

Since there's not a lot of info in your post. I would hang some string and a sinker from the ceiling at 40 inches to get an idea. At the height can you do what needs to be done? Then lower it.

Feel free to send me a photo using ASK TOM.