How long is a full day in Assassin's Creed, and is there a way to find out what time of day it is in the game?
There is no passage of time in Assassin's Creed that isn't related to a quest or a cutscene. I don't recall specific examples anymore, but I think the first game didn't have any night-time scenes at all and AC2 had a couple quest-related ones.
Still, there is no time of day at any given time, for lack of a better term, or a way to determine it. You can spend any length of time running around doing stuff and it could still be the same "day".
I believe OP is interested in the tip that suggests one should plan missions according to guard shift changes.
This mechanism has changed over the games, in AC1 you could just jump into the mission as you pleased and hence to do that at dawn/dusk would align with this. Just look towards the sun from a safe spot and wait for it to go down.
Laters ACs had the funda of accepting missions, in that case, time is locked and doesn't change for the duration of the mission. This, however, doesn't change the fact that you can do some prep work at dawn/dusk, like taking out archers etc. If the guards are not alerted, the dead ones will not respawn ( sorry, couldn't find references ) when you accept the mission.
Anyways, the way to determine the time of day is to stare at the sun for a couple of minutes.