Welcome to Science Buddies! This is a really great idea for a science project. Your questions are good ones. Bacteria are present in the mouth at all times so the data points could be collected at any time. If you are measuring before and after samples, it takes about 20 minutes for bacteria to double in number, so waiting at least an hour after drinking the beverage could give measurable results.
I recommend doing a small pilot experiment and measure bacteria at time zero, one hour, two hours, and after overnight to see what happens before doing an experiment involving friends. Also, definitely have your daughter personally supervise data collection to help keep experimental conditions controlled as much as possible. It would be helpful to have a detailed written protocol before doing the pilot experiment.
After the sample is transferred to an agar plate, the bacteria will grow overnight if the plates are incubated at 37 degrees Centigrade (body temperature), or within 2-3 days if incubated at 20-22 degrees C (room temperature).
One precaution before continuing. Has your daughter discussed this project with her teacher? Rules for doing science projects involving growing unknown bacteria have changed recently and local rules vary, so you do need approval before doing the experiment. Here are the rules for doing projects involving potentially hazardous biological agents. I think that if the agar plates are sealed and placed in plastic bags before incubation and never opened, that it would be acceptable, but definitely do check with the teacher. http://www.sciencebuddies.org/science-f ... ents.shtml
And, there are more rules for doing projects with humans. You will need to get a signed consent form from the parent or all of your daughter’s friends before doing the experiment. http://www.sciencebuddies.org/science-f ... ects.shtml
Good luck. Please post again if you have any more questions.