Aslong as you are invited and contribute in the way you do I don't see that there would be much of a problem. Clearly though this question is of concern. I then would simply ask and exactly as you have posted it here, "Am I overstaying my welcome?"

So I am thinking of situations I have been in where someone in fact was overstaying their welcom. It was when my sons were older teenagers. Our house seemed to be that house full of teenagers. Well the friends of my one son were gracious and I enjoyed having them over. They were polite and always quick to lend a hand - even with preparing dinner. The friends of my stepson however were users. Never once did they offer to help or do anything. What's more they expected it be done for them. Well I wouldn't have that after a while and simply told them off and invited them never to come back again.

Again, since you are obviously in the first category and nothing has been said to you I would assume you are on a positive footing. Keep up the offering to help. It goes amazingly far.