How is the reward for ongoing charity calculated?

The reckoning of perpetual charity is known only to Allah, for He alone knows the extent of its benefit. For example, if a person digs a well and dedicates it as perpetual charity, how much reward is there in the purification of those who cleanse themselves, the breaking of fasts for those who are fasting, and the drinking of humans and animals from its water? And if a person endows a mosque, how much reward is there in the prayers of those who pray within it?