system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5 at what generation does a royal become a commoner?

Prince Harry has children. Those children will have children and so on. How many generation is it that the mother/father is royalty but the kids are not and considered a common folk?
gpt
It depends on the country. Some countries have extensive royal families (and nobility who are not themselves royal but also not commoners) such as the UK.  Others, such as Japan have very limited royal families. 

In the UK there are basically three levels of society. The nobility (ie people with titles), the gentry (people without titles but above commoners, historically land owners), and the commoners.  The rules on who goes where is somewhat complicated but you can look them up if you want. 

As for the British Royal family, the King/Queen can generally decide to grant titles to whomever they want, but custom and societal pressures do place some limits.  Since a decree by King George V in 1917 the children and grandchildren of the current sovereign have the right to the titles of Prince or Princess.  So outside of the main line of succession you’ll lose the automatic right to a title of Prince or Princess after three generations. The great grand nieces and nephews of Charles III for example would not be entitled to Prince or Princess by that rule.  

However, members of the British Royal family may (and many do) also hold other noble titles (such as Duke, Earl, etc) which may or may not be hereditary and pass down to their heirs or entitle their children to Noble titles.  The rules for these are quite complicated and differ from title to title and realm to realm (England, Scottland, Wales, etc). 

Finally, while they may not automatically gain titles, descendants of previous monarchs may remain in the line of succession to the throne.  There are, as of last count over 5,700 living descendants of the British Royal line, some obviously very distant from the current main line.  That may seem like a lot but one need not actually look very far to find the first person in the line of succession who does not currently have a title and is thus technically a commoner, and that would be Prince Harry’s son Archie, currently 6th in line to the throne.  There is some question remaining of whether he and his sister will become Prince and Princess per the rules of George V (Charles III can change them), but assuming they do get titles it’s only a few more steps to the next current commoner, #10 in line Sienna Mozzi, daughter of Princess Beatrice, grand-niece of Charles III.