一般应该提前多长时间预约？

It depends on your visa type and the consulate you are going to. On each consulate's website, there is a link for estimation of waiting time.

Beijing

Typical Wait Time (Calendar Days*) for a Nonimmigrant Visa Interview Appointment

Visitors Visas: 30 Days
Student/Exchange Visitors Visas: 4 Days
All Other Nonimmigrant Visas*** (Excludes: A, G, K, and V): 30 Days

Typical Wait Time (Workdays**) for a Nonimmigrant Visa To Be Processed****: 1 Day

Chengdu

Typical Wait Time (Calendar Days*) for a Nonimmigrant Visa Interview Appointment

Visitors Visas: 3 Days
Student/Exchange Visitors Visas: 1 Day
All Other Nonimmigrant Visas*** (Excludes: A, G, K, and V): 4 Days

Typical Wait Time (Workdays**) for a Nonimmigrant Visa To Be Processed****: 5 Days

Guangzhou

Typical Wait Time (Calendar Days*) for a Nonimmigrant Visa Interview Appointment

Visitors Visas: 14 Days
Student/Exchange Visitors Visas: 1 Day
All Other Nonimmigrant Visas*** (Excludes: A, G, K, and V): 1 Day

Typical Wait Time (Workdays**) for a Nonimmigrant Visa To Be Processed****: 2 Days

Shanghai

Typical Wait Time (Calendar Days*) for a Nonimmigrant Visa Interview Appointment

Visitors Visas: 20 Days
Student/Exchange Visitors Visas: 7 Days
All Other Nonimmigrant Visas*** (Excludes: A, G, K, and V): 20 Days

Typical Wait Time (Workdays**) for a Nonimmigrant Visa To Be Processed****: 2 Days

Shenyang

Typical Wait Time (Calendar Days*) for a Nonimmigrant Visa Interview Appointment

Visitors Visas: 3 Days
Student/Exchange Visitors Visas: 3 Days
All Other Nonimmigrant Visas*** (Excludes: A, G, K, and V): 3 Days

Typical Wait Time (Workdays**) for a Nonimmigrant Visa To Be Processed****: 2 Days