Usually how long are the contracts in China for teaching English?

It will depend on the location, but usually if you are physically going to another country they will expect a minimum 1-year contract.
There are some summer teaching opportunities that come up, and occasionally a school will accepted a shorter position, but these are rare.

The first 3 months will go slower, then usually teachers find they have been there 6 months and their one year contracts are almost finished. Majority of the teachers plan on just coming for a year but the year usually goes faster then they think and they end up renewing and staying for more years.

In China, following most western countries, the school year starts in September. Which means that if you are hired to start September 1st, it would be a 1 year contract ending at the end of August. In Korea the school year starts in March and in Japan the school year starts in April.

Some schools that close in the summer will want to offer a 10-month contract so that you aren’t paid for the summer. it is important to note the holidays (paid or unpaid) when you are looking at what schools offer. Some teachers are happy with a 10 month contract as it can be hard to get a contract that would give you the summer off paid, and at least you have a couple months to travel which is appreciated by some.

If you are hired to start earlier/later, in the school year, then you may also be offered a contract that will end at the end of the following school year (so over a year).

Most schools want their teachers to renew and stay longer than one year, so it is rare that a school will not renew. So long term employment is possible.