I read this after I gave my 'one method' response. I normally do downcounters rather than upcounters. At a guess, I dare say you upcounter rolls over to 0 every 256 ticks (if it is an unsigned char)

The way I would normally do a timeout counter is:

unsigned char timeout;

if (timeout)
{
timeout--;
}
else
{
//timed out
}

This way the timeout can never 'roll over' - it goes to zero and stops. The main reson to count down is that most if not all cpu's can detect 0 very easily. Detecting a certain value requires a compare or subtraction.