It depends on the Sim, really, and how likely I am to complete their Aspirations or career goals alongside the heir's family.
My first 2nd-Gen spare, Russell, was married and TFB'd before moving out. They had two sons, though technically the second was illegal because I had to switch to their household and initiate it. But I did it for the sake of testing, so I'm not feeling too guilty there.
My second 2nd-Gen spare, Van, also married. However, I loved him and his wife so much, I let them live out their entire lives and complete all Aspirations and Career goals in the main household. They did not have any children, but it was more because I felt they wouldn't want them than anything else. Their graves will stay in my graveyard.
My third 2nd-Gen spare, Ray, I didn't care for much as he was growing up, alas. He was the adopted son for his father's aspiration and not being eligible for heirship meant I didn't pay much attention to him.
I thought about getting him married, but he was always kind of a loner. Ultimately, I had him adopt a son of his own and moved them out. That son, Beckett, comes over all the time and has been good friends with all of Generation 3 and 4 so far. Never saw Ray again.
My first 3rd-Gen spare, Veronica, is a sassy, hot-headed lady keen on mischief and a criminal career. My intention was for her to top her career and move out, but she's sooooo slooooooooooooow about it, she may end up staying after all. Plus, if she leaves, who will take care of the garden?
My second 3rd-Gen spare, Helena, I just plain didn't like. No particular reason for it, she was just always being overshadowed by Veronica. I moved her out straight away, though she still comes over often. Given Veronica's tendency for mischief and mean interactions, she's Veronica's only actual friend.
And that's pretty much it so far! 4th-Gen are still children and being born, so who knows?