How much of their wealth should a Muslim give?

2.5% of the wealth excessive of one's needs that remains with a person for one year provided this amount equals or exceeds about 7.5 tola of gold (exact amount may be seen in books) is obligatory to give to one's poor relatives, neighbours or the needy. That is the 5th pillar of Islam called Zakat.

Anyhow, the Muslims are encouraged to give maximum amount they can afford in charity.