In my experience there isn't a set time as it depends on the individual spirit. Some spirits reincarnate, some don't reincarnate. It very well could be that if you had children in a previous life that your children, grandchildren, extended family could still be alive.
In my experience they are given a choice, some choose to reincarnate others do not wish to do so.
Some are content with remaining in spirit because they feel their work on Earth, Universe, etc. is done as you've mentioned, others simply want to remain in spirit.
Others if given the choice choose to become guides to others.
Many believe that a soul has to reincarnate until he/she evolves enough or that a soul has to reincarnate, become a guide, etc. within a specific time frame; in my experience neither is the case.
Example : A person who passed away in 1235 A.D. remains in spirit until deciding to reincarnate in 2016.
Or a person who passes away in 2016 transitions into spirit, but after a short while realizes that his/her loved one is having a really bad time & after careful consideration sends them a familiar to help ease their grief.
A familiar can be an animal, plant, human, etc. it can even be the deceased loved one who chose to take on the form of the animal, plant, human, etc.
Example : A wife passes away & the spouse is beside himself/herself & cannot be consoled, etc. all of a sudden out of the blue a stray dog, cat, etc. appears & ends up becoming part of the family helping the spouse, children, etc. to cope & at times the family members will say something to the effect of "Wow, that dog, cat, etc. sure reminds me of ...."
Everyone has their own experiences & beliefs, these are simply mine.
I hope this helps to answer your question.