Appropriate length for dock lines

[seawolf]

Hello all,
I just got a good deal on 100 feet of 3 strand nylon and will be cutting it and splicing some eyes for new dock lines.
What is the typical length of line in comparison to boat length.
A general rule? Same length of boat? , or a percentage ratio?
It seems when docking and rafting up sometimes, the standard store bought dock lines are to short of length for the smaller diameter lines.
My LOA is 29 feet.
Thanks.
Bob