How to convert high fps videos to normal? How long it will be?

I'm interested in slow motion videos and there's something i need to know about them. As far as I know, the higher the fps, the slower the image will be right? But my question is how slow? For example, lets say I shoot a video 120 fps, 2 second video, how long it will be in normal speed? And one last question, can normal phones such as iphone can take slow motion videos? I heard an app but I'm not really sure about the quality. I know I'm not really good at these things but I'm trying hard to learn. So I'll be really thankful if you could answer my questions.

So… Normal NTSC standard frame rate is 29.97 frames per second. For math ease, most folks just refer to it as 30 frames per second. I presume this is what you refer to as "normal".

If you captured 2 seconds duration @ 120 frames per second = 240 frames.
240 total frames divided by 30 frames per second = 8 second playback.

If you had a Phantom Flex to capture 1, 000 frames per second for 1 second = 1, 000 frames
1, 000 total frames divided by 30 frames per second = 33.3 seconds or just a little over 1/2 minute.

BUT, when capturing the slow motion, you probably would not use all of the capture duration. Usually, the action is really fast and 1 second is a LONG time!

When capturing in high frame rate, be sure to have LOTS of light on the subject.

We don't know what you are editing with. Most decent video editors can adjust the playback speed, too. It will not be as dramatic as the high speed slow-motion discussed above, but when video captured at 30 fps is slowed to around 15 fps, it is not too bad. If you get slower than this, the video starts to get a bit jumpy.

Can "normal phones such as iphone can take slow motion videos?" It depends on the iPhone, whether it is running the right app and whether the app will do what the developers say it will do.