Most of my GAs are for longer than 1 day, since people live in different timezones and have to sleep and work.

always depends, quality of game/ level claimed for the giveaway / public or private(if you make it publc you don't care who joins) (if you make it private you wanna make sure people has time to join) / taste of the host of the giveaway / weather / ...etc

to me, it definitely depends on each and every single giveaway!
I like to make flash, 1 hour giveaways for games open to everyone, often of little value but still that can be enjoyed - I have LOVED so many free or 1€/$ games that one can't even imagine!
I tend on the other hand to give at least 3-5 days for the "bigger" giveaways, no matter if they're for whitelist, my 3 groups or lvl+..
Some giveaways I love, I tend to make them 2 weeks long in order to give everyone the chance to join and participate! :)
It definitely depends on so many things: the game itself, the restrictions I want to make (whitelist only, whitelist + groups, groups only, lvl 5/8+ only, open to all etc. - I don't make invite-only giveaways though) and so on..
Anyways, the flash giveaways I make are always open to all =P
Though, it's definitely better (if it's an important game of some kind) to give weeks of time to me!
Some days are fine if I'm giving away some bundled game and such.. but if I want to make a giveaway for a group of mine, of course I need to gove everyone the chance =P
So it's better in my opinion to think on each single giveaway about its value and its public and most of all live the website in a free and healthy way <3

long: exposure, events
short: farm cv
no giveaways: leech mode

whatever you want. ¯\_(ツ)_/¯

For public, I like to give people a fighting chance so 10-12 hours. I do try to alternate time zones so both hemispheres get coverage.

Public: at least one full day.
Whitelist/group: usually a week.
Invite-only for forum threads: longer than a week to support the thread.

i only doe 1h flash giveaways , so the people that really want it, have a reasenable chance to get it

I set everything at 1 day or longer, usually 1-2 days. I believe 1 day to be mandatory for fairness due to timezones given, 2 days is optimal, I dislike longer giveaways and rarely make them (unless events or special dates).

The shorter the giveaway, the more bots it attracts due to low entries, rather than humans who are doing something else.

4 days to one week. Give a chance to as many entries as possible.

Depends on what you wanna do:

1. You want attention for whatever: Big title, 4 weeks, we see that happening every day.
2. You want to include as many people as possible: 48 hours to 1 week is appropriate.
3. You want to be fair, but you don't want too many entries so the chance for each individual stays nice (and that's how I roll); I always start "now" and set the end date to 23:59 of the next day, so it's always between 24 and 48 hours.
4. You want to troll people / make bots happy: The lesser than 24 hours - the more bot percentage.

Around 35-40 hours is a decent range. Long enough for most people to see it, short enough to not sit on the site for ages.

Well, for public ones. if you run something on the forums, at least a week is advised.

Humble Bundle makes me anxious, so what comes from there usually has a short duration.
With the keys of the other stores from one to four weeks.

One day suffices. Gives everyone a chance around the world

I frequently do 1-2 hours and almost always under 24 hours. Often I just want to get rid of a key.