I'm thinking about grabbing a planet or two from some other players. I'm actually not interested in taking anything that's being used by an active player so I'm watching to see if they're leveling up. Is there an amount of time that is considered a "fair" waiting time? I see some guys in the low hundreds (some even below 40) that haven't leveled up in over a week. That seems kinda long to me, like a dead account.

Yeah, I usually wait a week, especially if they're not in a Legion or get kicked out of the one they're in.

Pssh. Waiting for inactivity is for cowards.
If you wanna take a planet, just take it and be prepared for the avalanche. It's much more fun. Honestly.

Not if they're hundreds of levels below you...

I turn the tables around and think about how I'd feel about the situation. I don't get upset if I get hacked or attacked, sometimes I'll return the favor. Seems like no big deal to do some badge farming. But if I started losing some of the few planets I have? That would be a lot harder to deal with and really sap the fun out of the game. So... not something I want to do to someone else. Maybe when I've got a few dozen planets and am going against others of the same level.

Okay, here is the difference.

If you are taking a planet from a high ranked player your legion will start to lose planets. (200+)
If you are taking a planet from a lower ranked player you will be fine. (199-)

Your wrong off the bat xD from you say 200+ is high ranks. There is NO way to determine what a "High" Rank is because your leveling up in ranks. I was level 78 thought a level 150 was real good. Nah level 119 and i can kill them pretty easy same if your level 300 and a 400 takes your planet. But with the high ranked anf low ranked in general of what your saying, nah your legion wont lose planets unless its awesome.

If you see a planet you want to take and you are able to do it... then do it! This is a combat game. The only exception is if the owner is a MUCH higher rank. Then they will just take it back, boost the defense, and you wont have a chance again. Unless you can flux it.

I just take planets as I please, and the owners haven't retaliated (yet). usually the ones managed y people who haven't ranked up in a long time or just put noob buildings or the wrong buildings on it, like I've seem a very rich mining planet and sparse research, guess what? the owner had no mining structures and only a few research structures! and a gas planet where the previous owner put builds that you get form artifacts on it, complete waste.

Use a governance probe...

I can see the honor code some people have about not taking 'active' planets. Don't slam the player for wanting to be nice. That'll pass...

If they do not gain a level in a week they're not playing, or at least, not playing this week. If they're not in a legion, I'd give them 2-3 days max.

But what others have said is essentially true - you won't have a choice later on but to take 'active' player planets, if you want good planets. There may be a lot of inactive players, but they are often lower level and have minimal planets with a rare chance at a good one.

but if they are hundreds of levels above you..... like me taking from a rank 700