I think this question was decided by SCOTUS in Petrella v. Metro-Goldwyn-Mayer, Inc.:
In a 6-3 ruling, Justice Ginsburg declared that laches cannot be invoked as a bar to pursuing a claim for damages brought within §507(b)'s three-year window. However, in extraordinary circumstances, laches may curtail the relief equitably awarded at the very outset of litigation,
>In common law legal systems, laches (/ˈlætʃɪz/ "latches", /ˈleɪtʃɪz/}; Law French: remissness, dilatoriness, from Old French laschesse) is a lack of diligence and activity in making a legal claim, or moving forward with legal enforcement of a right, particularly in regard to equity. This means that it is an unreasonable delay that can be viewed as prejudicing the opposing party. When asserted in litigation, it is an equity defense, that is, a defense to a claim for an equitable remedy.
Would the latches doctrine really apply to separate enforcement cases though? My understanding is that doctrine would prevent me from enforcing my rights against a particular actor if I wait too long to enforce those rights… but waiting too long against one actor wouldn’t be relevant if I seek to enforce those rights against a separate actor in a separate case.
You have the correct instinct. Laches (not latches, it really is a different word) is about a specific cause so it only helps if you've got a reasonable case that the plaintiff should have noticed this cause ages ago. If the cause is new that's obviously not going to work.
In a 6-3 ruling, Justice Ginsburg declared that laches cannot be invoked as a bar to pursuing a claim for damages brought within §507(b)'s three-year window. However, in extraordinary circumstances, laches may curtail the relief equitably awarded at the very outset of litigation,