Slot machines operate on the c.variable ratio schedule of reinforcement. This means that there is a random and unpredictable amount of time between each reward (a win on a slot machine), as long as the person keeps playing the slot machine.
Interesting question- slot machines are actually based on a degree of randomness in each individual play, but the payouts when those random events produce a winning combination are controlled to ensure that only a certain percentage of money played is paid back. Typically in the high 90% range.