This would depend upon the type of UART or Processor you are using.
Most stand alone UARTs have a break detect function built in.
Most simple Microprocessors do not have a break detect function ( 8051 and Microchip Pic families as an example).
In the 8051 style processor one would use the FE (framing error bit) to indicate a badly formed data byte.
One would then check to see if the received byte was all 00h.
This would indicate a break.
It is really unnecessary to time the break length as any received break (or framing error) would cause one to restart the packet receiving process .