True XD

It is generally because a public GA do not need any announce.
So I expect a GA from a post will be a invite only, but it is not necessary.
Just do the GA that you want to!
For the duration, generally most people like flash GA, just do it as fast as possible!
Just remember that a invite only GA need 5 (or more) entries for counting as your contribution value.

Time does not matter as long as the giveaway (GA)is public. (setting it at a certain level or region does not matter either).

But if you make it for (steam) groups/ whitelist/ inviteonly. You need to have at least 5 people enter the GA to gain CV (contribution value, the xp too gain level).

If the time is short, nearly no one has a chance to enter. Best is to set it for a day or more if you use the group, whitelist or invite only.

For invite only you need to post the link on forum or no one is able to enter. The others does not matter because they are autmaticly placed in the general GA list. That everyone sees taken they meat the requirments that you sellected (group, whitelist, level)