Reading and processing a PWM on AtTiny45
You don't need to read the duty cycle, just the time that the PWM signal is high. First wait for the input pin to go high, then start a timer, wait for the signal to go low again, and finally read the timer value. If other tasks need to be performed concurrently then use pin change interrupt to detect the high and low transitions. 

If the ATtiny is running Arduino software then the solution is even easier - just use the pulseIn() function. 

This technique works for all PWM values except 0 and 255. With these values the pin stays low or high permanently, so there are no transitions to time. The easiest way around this is to simply avoid transmitting 0 or 255.

Alternatively you could pass the PWM signal through an RC filter to produce a 'DC' voltage, then read it with the ADC. However this will slow the response time down, and the signal will produce intermediate values as it goes from one voltage to another. To get good resolution the RC filter would need to have a long time constant, and you might have to average several readings to get a stable value.