The money, permits and a local government-owned construction company are all in place. China can build large factories in weeks. Tesla built an entire general assembly line in 3 weeks when they were under pressure to increase Model 3 production.
Online ordering for Tesla cars in China has started and the configurator is quoting first deliveries in March 2019. Those will probably be shipped from the USA. However, rapid construction of a building and putting a first assembly line within 90 days seems within what Tesla and China have done in the past. Many Tesla analysts are not expecting the first Tesla cars from the new China gigafactory until the second half of 2019.