alright so if we’re all in agreement that Kazui is between 3-5 years old and Ichika is around 7ish, and the end of the manga is 10 years in the future, this all kinda works out into
Renji and Rukia get married three years after the end of the war, which means they immediately conceived Ichika after getting married. since. 10 minus 3 is 7.
We don’t know when Ichigo and Orihime started dating/got married for sure. We don’t even know if they conceived Kazui after they got married (maybe they didn’t even do that, maybe they just boned and it just sorta happened). We can just assume that they had Kazui two years after Ichika was born. So, maybe they got married one to two years after renruki and, like renruki, immediately conceived their kid.
or. kazui was conceived before marriage. i don’t know.
edit: brah they probably only dated like two years and got married, what t he fuck