My kids are in 2nd and 4th grades right now. I voted less than 5 days.
I usually have no problem tacking on 1-3 days onto a school holiday if necessary due to itinerary (a cruise) or cheaper airfare. Up to 2 vacations a year. I try to limit it to under 4 days total.
Last school year, they missed 1 day for an out of town family wedding (took Friday off when they already had the Thursday off) and 2 days for Disney after Spring break. DH flew down on a Friday evening after work to join us and we all flew back on Tuesday after the break when airfare dropped dramatically.
I try not to take them out too many days because inevitably, they will get sick too and miss some school. I think between the scheduled off days and the actual sick days, they missed just about 5 days last year.
The year before, they missed 2-3 days after Easter/Passover because we were on a 12 night cruise. The same year, they also unexpectantly missed 2 extra days because our Sunday afternoon flight home from Disney during President's Week got CANCELED while we were at the airport and we had to fly back on a Tuesday early morning flight.
This school year, we have another Easter cruise planned for 2009 and they will miss one day of school because the Monday flights home from Miami were so much less money. So far, that is the only day they will miss for vacation. Being sick is another matter...