PulseIn timeout reading a proximity Sensor

I need to have a maximum value of 454,708 microseconds (which gives me a 1rpm resolution on my sensor disc. Everything is wired correctly and I get values until I slow it down then it starts timing out.
Using an unsigned long variable. Do not have teh code with me that I used on the PulseIn fucntion, but I recall the documentation was sparse on the formatting.

Thanks,
Scott