Looks like it should give you 10-20% duty cycle as it is drawn. I'm assuming you're using a 100mA relay or close to it, say a Bosch automotive cube type like you'd find all over these cars. In fact, the circuit looks to me like it might actually allow two or more sweeps at the longer delay settings. Can you try tweaking the 100K value upward if you are using a low current relay? Gain in the PNP device may be a variable too. It is a really (relay) nice and elegant design.