The thicker the shell, the stronger your object will be. However, complicated designs don't need thick shells. For cookie cutters, the default . 8 mm should work just fine.
If it's one-time-use, both ABS and PLA are perfectly safe for use as a cookie cutter. The "food safety" of 3D printed parts is fairly controversial. In fact, whether any particular material is approved by regulators (such as the US FDA) for food contact is much more complex than most people realize.
For the most part, 3D printed cookie cutters are food safe because they are not in contact with the cookie dough for very long and because the cookie is baked after it is cut, which kills any bacteria.
I've recently found printing my cutters on draft/standard results in a good cookie cutter quickly. I've been printing 3-4in. cutters on draft at about 20-38 minutes. High quality I've seen go as long as almost 2 hours for a large cutter.. . . . .364011234505!]0000000000000009783314145882!]
