i go with what looks good to me. if adding more than one border, i put a skinny one next to the center

Lately I have been noticing some very wide borders (20" +/-) to cover the new deep mattresses and/or to minimize the amount of piecing you have to do. I guess this is when you are really after a bedspread. The other replies are all good.
For my bed quilts I was adding on average 6" borders, but think I will be increasing some of them to 10" for the same reasons.

Beginner and I should had cut my borders at least a 1/4" wider & longer to accommodate for squaring up. Because I can't sew straight, once I square up, I'm going to end up with a finished border (when quilt is finished) of 3/4" border on an 8"x 10" block. (custom memory quilt for son) I've never seen an inner border this thin, lol. Lesson Learned

I like big quilts. My DH is a blanket theif, so if I made my quilts the size most people post, I would never get any of it. So I make my king size 115 inches wide. I make the center of the quilt whatever size the pattern works out to, then add what borders I like to get the width up to 115". When we had a queen bed, I liked 96-100". It is your quilt, do what makes you happy.

I like to use the blocks as my guidelines and use a size that corresponds to the blocks. If my 12" blocks have 3" segments, I can go with 1 1/2" for inner border and 3" or 4.5" for outer. (Or something like that.)
Other times, the amount of fabric I have left over determines the size of the border. If I don't have enough to cut 4.5 it will simply be smaller.
For smaller quilts in particular, the availability of backing fabric may determine how big I will make the top (and the border). Not sure there really is a rule. It's all visual impact and personal choice for me.