We also don't duplicate signatures. If we already have Mickey, he doesn't sign again. (per trip that is)
In restaurants, it also depends on how crowded it is. We've hung out with Goofy several occassions in Cape May Cafe, the restaurant was practically empty. Goofy sat and colored with my son for at least 15 mins!
We also try to spread out the meal so the characters can stop by our table at least twice. First round, do autographs and photos, second round play.
If you have the autograph book ready (page open, pen ready) they usually will sign, pose for pictures and interact a bit.
It also depends on how much your child wants to interact with them. At Hollywood and Vine, a girl at the table next to us was so afraid of the characters that they lingered a bit longer at our table.
We also have our favorite characters that we interact with more than others. My DS4 has a huge crush on June, so when she stops by the table we tell her how much he loves her, that this is the highlight of his vacation, etc... She makes a huge fuss over him.
I wouldn't not get the autograph and photos. Some times, this is your only chance. In peak season, the characters are very limited outside and the lines get very long.