In generation 5 and below, Drought, Drizzle, Snow Warning, and Sand Stream all last forever unless a conflicting weather starts or a Pokemon with Cloud Nine comes into battle.
In generation 6 and probably up, which I assume you're talking about because you mentioned Mega Charizard, Drought, Drizzle, Snow Warning and Sand Stream only last 5 turns, and can also stop for the reasons listed above.
Source: Knowledge