It is different depending on the communication conditions, but the basic point of view is as follows:

In ASYNC, the sampling is performed at the clock of 16 times baud rate. It centers on 8th clock from the point of the falling edge of the start bit being detected, and after that it performs the sampling of each bits in every 16 clocks. The error range will be the range which does not shift from an original bit of the sampling timing (within half bit).

Example: Total 10 bit [Start bit (1) + Character bit (7) + Parity bit (1) + Stop bit (1)]

The error range will be ±0.5/10=±5%. In fact, it becomes still smaller depending on the communication distance and the performance of a driver and a receiver.