I know that a longer boil will give me a more potent wort and slightly raise my OG, but I was wondering if a longer boil also impacts the fermentability of my wort? I am wanting to use a 2 hour boil on a big SMaSH beer to give some more depth of character, but do not want it to finish high and be cloyingly sweet. Any input or experience here?
mash temp, grain bill, PH would all be things to look at to drive "depth of character"...but you'll have to define what that means to you for more help.
boiling longer is recommended on some beers to develop 'depth of character' - doppelbocks, barleywines and scotch ales are some that come to mind. the thought is a more caramelly or melanoid-y character is developed (although there is the argument that neither of those reactions actually occur since caramelization occurs in the absence of water).
anyhow - I don't know that 2 hours is going to really be noticeable to be honest. most of the aforementioned styles, if the recipe calls for long boil times, are usually in the 3-4+ hour range.
for me personally, I spend 6-7 hours on a brew day as it is, and my standard 90 min boil has always been enough for me. Adding 2 more hours to that better be noticeably better, which longer boils and decoctions have not proven themselves to be.