It depends on the content of the topic and the category in which it was written.
For example, in the support category ("I have a problem, help me!!1!) it is useful to close the topic after a post has been marked as a solution. The time depends on the nature of the forum itself, it could be a few hours or even months.
Here on Meta we do not leave open solved topics, but we have adopted a very simple system, that many users adopt, they flag to contact the moderators asking that the topic X be reopened because they want to add / modify / ask something. This system works very well for us.
Some topics are closed obviously when they run out of function, or they become obsolete because they are too old, or because they are extremely controversial (we like lively discussions if they have a purpose and when remain within the limits of “criticizing the idea and not the user”, otherwise it is better to close them).