Rewards are dispensed at various predetermined intervals of time. Faucets usually give fractions of a bitcoin, but the amount will typically fluctuate according to the value of bitcoin. Typical payout per transaction is less than 1000 satoshi, although some faucets also have random larger rewards. To reduce mining fees, faucets normally save up these small individual payments in their own ledgers, which then add up to make a larger payment that is sent to a user's bitcoin address.
As bitcoin transactions are irreversible and there are many faucets, they have become targets for hackers stealing the bitcoins.