Don't let it go more than a day or two. However, if the student missed part or all of the material taught, you should give him a deadline by which to make up all missed work, and a new test date. It may be helpful to contact the parent in this case. A student should not be penalized for being absent. However, they can be penalized for failing to make up missed work.
Students are most motivated to learn at the beginning of the school year.
Try to teach students good problem-solving skills. They will more likely have a scheduling problem that needs to be solved using LCM concepts. To motivate students, give out awards for both good academics and for good effort. Do your best to be fair to students. You will earn their respect this way.