When it says No More Heroes on the cover.
But seriously.
It depends on the game and how many elements it has.
Some games you can get a good taste of within an hour.
Other games (like RPGs and many other open world games: GTAs Fallout 3) become more engrossing the more you explore them.
With RE4, it probably wouldn't gotten any better for him. If it was San Andreas, I would've told him to try at least opening up the map.
Really though, No More Heroes is the only game I think is more overrated than RE4. RE4 was a slow sandbagged shooter with quick time events; it's no wonder I didn't like it, until it came out on the wii.