I bought nwn for multiplayer not realizing they don't provide a code usable for multiplayer and submitted a ticket last night. How long do they usually take to respond and provide with the necessary key?

Usually they are pretty quick compared to other stores. You shouldn't need to wait longer than 2 days, I think, unless half of their team is currently taking their summer vacation. ;)

I take it you've already looked at this post about submitting a request for the multiplayer code for Neverwinter Nights?

Response times for support tend to be quite quick. But note that support is in Poland, which is 7-ish hours ahead of you. So, since it's 18:26 in Poland now, and if you haven't received a message yet, they probably haven't gotten to it yet.
However, when you're asleep tonight - they'll be waking up and getting into work!
Fingers crossed you find a message tomorrow! :)

I had a problem which needs resolving (they owe me a refund), and submitted a ticket at the weekend. I haven't heard anything back from them yet, which is quite disappointing really seeing as they have had 2 complete working days to get back to me, even just to tell me that I need to wait a little longer.
*****edit*****
In the time between checking my emails then coming to the forum to complain about lack of said email, I have received a reply saying a refund has been processed.
I take back any negatives regarding gog and you can be confident they will get back to you, they must just be busy.