Oh man, I'm glad I saw this thread. I'm 29, I started playing goalie when I was about 6 and played straight for the 23 years (multiple times weekly, and daily for a good part of it). Through highschool/college/beer leagues I had been hit in the head probably hundreds of times. However, when I was 25 and playing pickup, I took a shot off the center of my forehead (as I had done hundreds of times before) and instantly it felt different than the others. I kept playing because I'm an idiot and I didn't really understand the gravity of concussions. After I got off the ice, I was a bit confused, nauseous, and quite sensitive to light. It took a few weeks to get back to normal but didn't have to do much else.

After that I started playing forward a bit more, and started skating about 5 times a week and playing in beer leagues as a forward. As I'm sure you know, once you get one concussion subsequent concussions are much easier to get. In any case, after skating out, and playing in some leagues with assholes, I ended up getting two more concussions over the next year.

So to answer your question, i don't think there's any more or less of a risk playing forward vs. playing goalie. It's more just a chance thing and if you get hit the right (read: wrong) way. And to answer your more specific question, what it takes can really vary. You can get plowed over in net, hit your head on the ice or the post and sustain a nasty concussion (luck would have it, that never happened to me). But you can take a shot to the dome and it can cause a concussion. I don't think it's common, and it took me probably 200 times before one caused a concussion. But any one of those shots, if your helmet is shitty, can definitely cause a concussion. So the lesson is, buy a good helmet and replace it periodically. It's expensive, but brain damage is even more costly.

It's not something you should be concerned with playing and taking occasional pucks to the mask. But should it happen, just get off the ice and rest up.