I couldn't find the explanation of reputation scoring over time. Does reputation decay over time?
No, there is no built-in reputation deflation. There are cases where reputation can be lost (e.g. a reversed vote or a deleted post), but there are no processes in place to systemically decrease your reputation over time. Once earned, reputation is yours to keep.
Every time I'm scanning the posts on meta, I read this as
Is reputation completely addi c tive
And I answer myself that yes, yes it is.
Added note to @RobertCartaino's answer: How does "Reputation" work?
This tells you "all" about reputation.