Here is a description of the circuit I posted:
The 555 Timer is configured as a retriggerable one-shot multivibrator. As long as the Sensor input is low, diode D1 conducts, discharging C1 to near ground. Because the voltage at the 555's Trig pin is below 4V (1/3 of 12V), this puts the 555 into the "triggered" state, where its output pin 3 is high, which causes the collector of Q1 (Output) to be low because Q1 is turned on.
When Sensor goes high (near 24V), D1 is reverse-biased, allowing R1 to charge C1 toward 12V (from the Zener). It takes R1 a few seconds to charge C1 to 8V (2/3 of 12V). This causes the 555's Thresh pin to put the 555 into the "reset" state, where its output pin 3 is low, causing Q1 to turn off, allowing R4 to pull the Output all the way to 24V. If the Sensor input goes low before C1 reaches 8V, C1 is rapidly discharged via R2 and D1, preventing it from timing out. Only after Sensor goes high and stays high for longer than the timing period will the Output go high...
24V exceeds the maximum allowed operating voltage, so I used a Zener regulator to operate the 555 from 12V. Since the Sensor input swing is from 0 to 24V, I returned the pull-up resistor on the output to 24V. C2 and C3 are bypass caps...