Thanks for the comments, much appreciated.
However, it's clear I didn't ask the question clearly. i 'chose' my words poorly.
Here's the question: when an experienced brewer looks at a recipe, what are the specific items that would make him/her say, "That probably needs some extra fermentation time?" Is it Safale S-04 vs Nottingham Ale yeast? Chocolate vs. Crystal 20L?
I'll ask this way, too - below are two recipes from 5 gallon kits from Northern Brewer. For the Irish Draught, they recommend 4-6 weeks in the fermenter. (2 weeks primary + 2-4 weeks secondary) For the Porter, 2 weeks (1+1). Looking at the recipes, I don't see why they have those recommendations. What is it about the Irish Draught that would need a longer ferment?
Irish Draught White House Honey Porter
OG: 1.051 1.062
IBU: 28 23
ABV: 5.32 6.53
Fermentables:
Light LME: 3 lbs Light LME: 3 lbs
Light DME: 2 lb Honey: 1 lb
Honey: 1 lb late addition
Steeping Grains:
Crystal 30L: 0.5 lb Crystal 20L: 1 lb
Golden Naked Oats: 0.25lb Bonlander Munich: 0.75lb
Chocolate: 0.25lb Black Patent: 0.375lb
Chocolate: 0.188lb
Hops:
1 oz Cluster Nugget: 0.5oz 45 minute
Nugget: 0.5 oz 30 minute
Hallertau: 0.5 oz 1 minute
Yeast:
Safale S-04 Danstar Nottingham Ale