I think the longest I was away from my children when they were young (11 months, 4 years, 9 years) was around eight days.
The longest I've been away from my children full stop (ages 18, 13 and 10) was nine weeks.
When my children were young, my husband worked in Europe but we lived in England. He was only in England for 70 days a year for over five years. On a long stretch of him being home, I would visit my family in the US hence the eight day break away from the kids.
I left my family for nine weeks because my father, who lived over 750 miles from me, had gone into hospice care and I stayed with him to look after him and to take care of his effects after his death.
Those were the longest stretches of time at once but we have left the kids with the other parent for days here and there throughout their childhood. Because I have never had any relatives nearby to take the children off our hands, it was definitely necessary to go away for a few days to have a relaxing break so we could come back to the family refreshed.