I use them when

1) I can't figure out what I need to do to progress and have been trying everything I can think of for hours
2) I don't know how to defeat a miniboss or boss, have tried several times with no luck, and need help strategizing
3) I want to know how to access secrets

Never, the closest I've come to that is asking TBD how to beat xenoblade bosses because she pwns them all.

I only use them when I am absolutely stumped. A few examples are the original zelda, zelda 2, etc.. But mostly I just screw around until I figure something out..

Walkthroughs are my last resort when all hope is lost. But I never ever ever ever EVER have used a walkthrough to help me out in a puzzle game, its a rule of mine to never accept help in puzzle games.

But when I'm done with a game, I do like to look at walkthroughs for any possible secrets and easter eggs I may have missed. But only when I'm done so no spoilers.

Never.

...unless it's Mega Man.

When I'm helplessly stuck. So, in other words, a lot.

i give it about an hour of trying to get past a certain point in every way I can imagine. once I have exhausted all possibilities I can think of, I will consult a walkthrough.

i use them to cheat all time...

I only use walkthroughs to get information if I've been stuck on something for a long time and it's gotten to the point where I'm not enjoying it anymore - pretty logical since that's the point in me playing in the first place.

1. To find valuable "missables" in games so I don't need to restart if I want 100% completion.
2. To determine drop/carve rates and other statistics not normally viewable in games like Monster Hunter.
3. To find out what enemies are weak/strong against what in some games when I can't remember but have done that part before.
4. To see what strategies the guy who wrote the walkthrough used to beat the boss after I've beaten it myself.

I use them all the time. I don't usually need to use them because I'm stuck, however.

I'm hopelessly stuck on La Mulana, but I refuse to use a guide. I think I'll go insane if I'm on it for another two hours trying to progress.

Otherwise, I've never really needed a walkthrough. Skyward Sword had me stumped, bu the solution was quite simple in the end.

Only time was when I got FF VII, I played through the whole game using the guide and it was still the most amazing game I ever played.

I rarely use guides unless I hear about cool content, etc. that I've failed at finding myself; like in Majora's Mask with all the extra side masks to get the deity mask. You need to be at certain places at certain times/specific time frames of the day; I'd have gone nuts if I had to find them all myself.

@Chriis that's literally what the bomber's notebook does.

When I wanna beat a game like a boss, and fast

Only when I am really stuck. After I continue I won't use it again unless I am stumped later on. I take a lot of pride when I can beat a game without using a guide.

I also check them when I have beaten a game or a level, to get to know about secret things.

Number 1 is wrong.

You should have written '1) After I create a topic on NintendoLife and get told by everyone to look at a walkthrough because we are not gamefaqs'.

If I've been stuck for more than 1/2 an hour on something really stupid. E.g. in Demons Souls near the start of the game when you have to find someone called "The Monumental" and they zoom into a giant statue. I think that's The Monumental, try for half an hour to talk to it, give up, go on the Internet and see the Monumental is some kid who blends into the background PERFECTLY.

And if watched Lets Plays of games which I haven't played yet but later played, and happen to remember certain things.

Also, I almost always use one on my second run of a Pokemon game to see what Pokemon are available where.