The vaccination schedule for tetanus varies from country to country but the principle is generally the same across the board. It’s important to remember that tetanus is a very rare disease. In the UK for example (population approx. 60 million), only 6 people get it each year.
The Principle
With all vaccinations, whether for tetanus or Hepatitis B for example, the principle is the same. You should receive the first few doses quite close together to encourage a strong reaction by your immune system. Later, you can receive booster doses years apart.
United Kingdom
In the UK, for example, children receive the vaccine at two, three and four months of age. They then get a booster at around 3 years of age and a further booster in their teenage years. This should give full immunity for life.
Adults who never received any immunisation get three shots at one month intervals. They then get a booster shot 10 years later and another one another 10 years after that. Again this gives immunity for life.
United States
The US has a very similar immunisation schedule for tetanus but also recommends a booster shot every 10 years for life. This is at odds with UK opinion. Having said that, there is no risk to having “too many” tetanus shots.
Combined Vaccine
It is quite rare to find a tetanus vaccine on its own. More often you will find them combined with any one or more of the vaccines for diptheria, pertussis and polio.
Risks
It’s important to remember that it is quite difficult to get tetanus, even if you are not immunised (some people cannot due to allergic reaction to the shot, for example). The bacterium responsible for tetanus is anaerobic and cannot multiply when exposed to air. Also, you can only get it if the spores get in your wound. These spores are not found in rust (a common misconception) but are found in animal faeces and organic soil.
So there is a higher risk of tetanus from a puncture wound e.g. from a nail (rusty or not) or from a wound that is visibly dirty where animal faeces or soil is present. A typical random scratch is very unlikely to give you tetanus.