Serial UART timeout

I need help on how to write a serial input timeout – the sample code in C that I need with 8051 microcontrollers. The input serial timeout is that if there is no key pressed from the PC for more than 60 seconds, then the program should display a timeout.

This sounds like a homework question, so we don't supply code.
One way would be to use the timer to generate interrupts every 50mS.
In the interrupt handler add one to a count.

When a character arrives clear the count.
If the count ever reaches 60*20, display the timeout message.