As a teen growing up, and as soon as we could afford it, my mother regularly got our house fumigated. Every few months, the exterminator would come in and spray the whole house - leaving us insect free for a while. One reason being that, at one point our neighbours were rat infested and those rats used to come through the roof and over the fence, into our house. Another, being her fear of roaches.
So the appearance of even one or two cockroaches in the house, meant it was time to call the fumigator again. Meaning that, I lived a pretty insect free life (and inherited the roach phobia myself).
Similarly, my apartment today gets extermination once a month as a service provided to tenants. So I've never really thought much of it. In fact, sometimes I find it a nuisance because I'm busy doing something when the exterminator comes. So last month when he came and I was busy, I told him to go away. I haven't seen a roach in months, and not more than 10 in years... until this morning.
Just walked into the bathroom stark naked, to see a big fat cockroach in the tub :O
I'm never going to turn the exterminator away again!