How to decide the level requirement for a giveaway?

Considering Humble has 1 dollar tier with bunch of games almost every week, I dont see a reason to say people cant afford to give away games. Its more of a mindset..
Ive given away little. But I still have had problems with low level users. If the game isnt anything much I would give it for a lower level. But usually I tend to give for level I am currently in..

do whatever you want. this site isn't charity, and not everyone deserves free games. ^^

find settings for your giveaways that make you feel good and stick to them.

Well, a lot ofpeople already worded my thoughts, but I'll leave you with my personal choices.
So, choose however you feel, and as has been stated 0 means bots and the likes, 1 means meh, 2/3 getting there 4/5 active 6(+) super active
People already mentioned cheap bundles, but there are even free ways to get ex-bundled games, so everyone could be lvl 1+
I usually use:
Level 0/1 for once-free games, as most people probably have them
Level 1 for DLC, as not many people might have the main game, so spreading the chances.
Level 2 for good games
And, although I haven't given AAA games (AFAIK), I would do those level 3/4 minimum

And note, that if you combine this with SGTools entry or regular Invite-only, you might want to lengthen the duration to actually get entries.
Also, I always check winners, and requeust a reroll in case of infractions. Together we can keep this nice community a nice community.

Level range just helps minimize the ratio of rulebreakers to non-rulebreakers that'll be entering your giveaway (mocked-up visual aid [with the intent to give a general impression of the rate of decline, not to give accurate numbers] below, you can click on it to enlarge).
It doesn't do much to affect other negative traits (like leeching), for which you'll want to use groups or SGTools filters to avoid.

The key to making public giveaways (and forum ones, though the decline in abusive behaviors comes a lot quicker there, with level 2 forum giveaways being more comparable to level 4 public giveaways) is to find that sweet spot where the amount of rule-breakers you end up encountering isn't enough to stress you out to the point where you regret your giveaway. For some people, they don't mind rulebreakers at all, or feel the benefits outweigh the stress, and they find sticking to level 0 to be agreeable. Others prefer higher levels and, for a number of us, we end up sticking to SGT or groups for most of our giveaways.

In short, as Mully notes above, it's all about finding what works best for you.

View attached image.

I like level 3 because I feel it avoids most troublemakers while still being easy enough to reach. I avoid too high levels because there are many people who reached those by only doing group-only GAs in tiny groups. And there are still some bots there too anyway, I'm almost sure I saw one at least once in a lvl 5+ GA.
But yeah, as others say, it's really up to you. If you don't mind the occasional trouble, I guess going for level 1 is good enough (as others said lvl 0 is pretty bot-ish I feel, and it's really, really easy to reach lvl 1, you just need to give like a 2 cents key from DIG).

I used to do 5+ ga, but most of the winners are still leeching hard so from now on 6>

I've never given at any level but 0. I do limit access sometimes by posting to a group (Unlucky-7, a group for those who haven't won much), and did some forum specific giveaways in the past.

People say that low level giveaways have more need to re-roll due to rule breakers, so those who give away a lot prefer to do higher levels just to reduce their work. I don't get that much and haven't been bothered that much (though I do get to reroll sometimes).

Other than this, I feel that level choice is a matter of outlook. The more cynical you are and the more into 'random trading' (i.e., giving to those who give the most) you are, the more incentive you have to do higher level giveaways.