Two weeks 5 days when they went long haul for a family wedding with their paternal grandparents. The girls have been going away for holidays with their GP's for years though. Same with DS1.
My youngest was away for 3 days recently with MIL and it was great to get a break, but very nervewracking as she has mutiple medical issues so I'm not as relaxed with her being away as I am the rest.
I think it's something each person just has to make their own decisions on. I know I'm a better parent for the breaks, and also I relax more knowing that if anything every happened to me and I ended up in hospital or away urgently they wouldn't bat an eyelid staying elsewhere. That was important to me because of my own childhood circumstances.