There are 4 years between every leap year. Correction: There are 4 years between MOST leap years. That's the case 99.25% of the time. For the other 0.75% of the time, there are 8 years between leap years.
It varies widely. Some volcanoes will erupt every few years or even every few months while others can go hundreds of years without an eruption. Even for one volcano activity is not regular. Major eruptions are often preceded and followed by a series of small eruptions.