I'm not sure if I have the normal size one or a mini, because they're not that big.  I keep one on my piano at the end of a hallway, and one on the desk in the famil room.  Every time someone walks by the sprayer goes off.  It won't go off again for half an hour unless you push a button.  There are several scents available and the cartridges last quite some time.  I've only replaced them once and we got them about 5 months ago. 

We got the units for darn near free by combining the plethora of coupons available online and in our weekly news paper with a sale at our local kroger store.