You are correct in that there is a point where you need to draw the line. You don’t want to do other people’s work for them, you just want to help them get on the right track.

In some cases, I have used a 2-minute rule. If I cannot help the person in 2 minutes, then it is obviously a bigger issue than a quick question. My current job requires some support for other groups, but we get to “charge” hours to their projects. In those cases, my hours are more expensive than theirs so they know to only ask questions to get them started. Budgets can help in your management of these types of situations as well.

I would like to add something to the point “helping your peers”:

If someone asks me for an information I can give immediately or after just a moment of thinking – I’ll be happy to do so. But if I would have to search for the information myself, I just tell them where I would search or who I would ask, instead of just doing it for them.

The important difference is:
If I give an information or a useful hint which helps them to finish their job, I am know as a helpful coworker who likes to share her knowledge.
But if I finish their job, chances are good that I am known as someone to who everybody can give their unloved tasks.

Thus, better think about in which way you help, because there are good and bad kinds of help.