Although there are many types of visa for China, the most common is the L visa for tourists, the M visa for business, and the Q1/Q2 for those with a Chinese partner and the Z visa for people with work contracts in China.
Visa durations are generally 30 days. You may be able to get an L visa with double entry, meaning you can stay 60 days in total, but, you will have to leave the country prior to the end of the first 30 days. Going to Hong Kong or Macau and back will suffice. Z visa is normally twelve months, although that may depend on the contract with your employer. I often hear of people getting multiple entry M visas. These durations and whether multiple entries were available was always in flux. You can only really only know for sure at the time of application.