I usually do out and back. My main out and back is 15 miles. So I do multiples of 15. I do 15, 30, 45, 60, 75, 90 (or stretch it to 100 if I am already doing 90) miles. My main ride being 30.

That way I keep bottles of water or recovery drinks at my truck and get fresh bottles every 15 miles without having to carry it all with me.

That is for road biking, not mountain biking, of course.