I use 16' of 3/8 BBB with a 40lb anchor on a 36' boat, but that is a little overkill. Upside I don't drag even in mud.
Tell us more, what size,type of boat?
Where do you plan to anchor?
What type of anchor? (don't start another anchor thread
), just traditional, spade, plow, etc...
What size of anchor?
On a 25 footer especially for day anchoring, you don't need an excessive ground tackle. But I would, (and do), keep a second storm anchor in a locker for unexpected storms.