I wouldn't expect to see Mega Pokémon for about another year. We will probably have 3 XY sets, THEN they unleash Mega Pokémon.
1. XY being a new generation is exciting enough to attract attention
2. 3 sets in might just be beginning to bore people, but Mega Pokémon will bring them back
3. Black and White did the same thing
4. It could make sense with rotation. Rotating all of the EXes before Mega Pokémon enter the scene
5. It would suck if Mega Pokémon are worse than EXes, but if they are better, then we could have a serious problem. So rotate so that they do not co exist.