It varies a lot depending on the specifics.
In the case of Predatory Rampage, we're talking about two continuous effects generated by something other than a static ability. One modifies the characteristics of objects, and the other doesn't. Both of those situations are covered by 611.2c.
611.2c If a continuous effect generated by the resolution of a spell or ability modifies the characteristics or changes the controller of any objects, the set of objects it affects is determined when that continuous effect begins. After that point, the set won’t change. (Note that this works differently than a continuous effect from a static ability.) A continuous effect generated by the resolution of a spell or ability that doesn’t modify the characteristics or change the controller of any objects modifies the rules of the game, so it can affect objects that weren’t affected when that continuous effect began.
- "Creatures you control get +3/+3 until end of turn." affects only the creatures that existed when Predatory Rampage resolved.
- "Each creature your opponents control blocks this turn if able." affects all matching creatures at all times.
Both of these are covered by rulings in Gatherer.