How long do the effects of Botox on the forehead and between the eyebrows last?

Lately, whenever I look in the mirror, I find myself bothered by the wrinkles on my forehead and between my eyebrows.
It seems like there are more of them than before, and it also seems deeper.
So I'm thinking about getting Botox.
How long do the effects of Botox usually last after a single treatment?
I am also curious about the interval at which the cycles should be aligned.