A circle is made of 2pi radians. I usually imagine that the mean-peak/ peak -down to mean etc is like a quarter of a circle, and 4 of those comprises a circle, so a wave takes 2pi to complete a full oscillation. Basically every time it goes from min to max is 180 degrees or half a circle (pi radians)
So when the peaks are the same, it's constructive. And this happens a full oscillation apart, so it's 2pi... A full circle. If it was pi radians out, then a peak would be with a trough, and it would be destructive.
So multiples of 2pi, or 360 degrees, will be constructive.
If it is pi out, then it is totally destructive
Numbers in between will produce a really noisy interference - At times it will be constructive, then destructive, then somewhere in between.