In my understanding most teachers are required to have their lesson plans completed way ahead of time usually by a whole month or at least two weeks ahead of time. So the teacher should already KNOW what the kids will be studying and working on during a vacation. The most I would probably keep my kids out would be 5 days consecutively per year and most teachers will already have their lesson plan prepared a week ahead of time.
Again I think it has a lot to do with if the teachers like you and your child and are willing to help out. Since my mom put a lot of classrooms hours in as a volunteer the teachers were more than willing to return the favor.
Just curious but have you ever had to change plans based off of someone else's (namely a child's) needs?? Teachers may have their lesson plans ready a month in advance but then several students to the majority of the class are confused and need more time going over the information. This results in the lesson plan no longer being valid and needing to be changed. On Monday we find out what my youngest daughter's class will be doing for that week of school and what her spelling words are. Since I know MY child I know how to prepare her for what they are studying and how best to help her. Not everyone else's child is the same as mine so I cannot say how well or what works for them. If the teacher feels they are not comprehending than he will slow down and find another way to teach the subject. This may cause my daughter some boredom but then she may also get more out of learning it a different way.
Kids don't all learn the same way. I have 3 different kids with 3 different learning styles and I have to adjust how I try to help them understand their assignments. One child is a visual learner while one is a tactile learner while the youngest has not yet really shown the best way she learns yet so I incorporate several methods for her. Just because all 3 of my kids are extremely smart and above grade level does not mean all 3 are always doing great in school.