I've had ant problems in my place for the last three years or so. The ants are extremely tiny and able to squeeze through the tiniest crack. They show up inside once the weather outside warms up and I think they've been getting in around the kitchen window. The first sign I get that the ants are back is a long column or two of ants marching across my kitchen counter.
I did some research online the first year I had the ant problem and I discovered that ant sprays and traditional insecticides aren't always effective or even recommended for certain kinds of ants. Luckily, I found a product called "Raid Ant Bait Double Control" that took care of the ants really quickly and it's cheap and easy to use as well.
Each package contains 4 ant baits. The baits are small plastic "feeding" stations, containing two different types of food to attract a wide variety of ants, that you place wherever you have an ant problem. The ants enter the bait and then carry little bits of this poisoned food back to the nest which wipes out the whole colony relatively quickly.
Each year, when the ants appear on my kitchen counter, I put one bait close to each column of ants. I don't use anything else. Once the ants discover the bait, they don't seem to be interested in anything else and within a day or two, the ants are completely gone - at least until the following year.
I had the kitchen window replaced in March of this year and the entire opening was spray foamed, so I don't know if the ants will be back this year.