Number of games in a series plays less of a factor in franchise fatigue than the content of the games themselves for me personally. I'm not planning on picking up Ascension, but it's more because of the direction and apparent content of the game than the fact that it's the xth entry in the series.
After God of War III I was ready for the series to go in a different direction, and Ascension just doesn't look different enough.
Of course, tolerance when it comes to stuff like this varies from person to person and even from genre to genre (for most people, I'd imagine).