Starting at the beginning of battle, both you and your opponent have 0 points in their Mega Bar. Each turn, you and your opponent gain 1 point each, capping at 5. When you reach 5 points, you can have the pokemon with their respective -nite megavolve. However, having the pokemon with the -nite in battle would increase the bar by 2 points every turn for the pokemon's side. If the mega form switches out, the counter for the pokemon's side is reset to 0, and the pokemon reverts to its original form. In-game items can give you a full meter, and will use as many Wonder Launcher points as Full Restores.
Tl;dr: a Slow Start for mega-evolution. --- Apparently, I'm nobody. B2 FC: 3010 5891 4441
From the leak footage we got, you press a Button and then choose an Attack and the Pokemon will Mega Evolve and do the Attack you chose for it. --- Currently owning Noobs on Pokemon Showdown and in BF3 on XBL Waiting for Pokemon X and Y(10/12/13)
Won't ever happen, because it contradicts existing information.
1) The Mega Evolution is button triggered, and seemingly avaliable from the start of battle.
2) No additional progress-bar type indicator is present on the battle screen.
3) Mega Pokemon stay that way, even when switched out. Only when the battle ends does the evolution revert. --- I gave up and did the 3DS transfer anyway... 3DS XL FC: 2492-4470-1418 / White 2 FC: 0047-4149-5185 / White FC: 0348-0626-6780