This may be strange...

Mine is 1 kb.

I try to keep somewhere between 200 and 250 gb free

I think if I dropped below 100Gb free on each of my drives, i'd start deleting things

Mine is 1 MB.

10% of the HDD size...since mine is 232 GB I keep over 25GB empty at all times.

its never even crossed my mind to keep free space tbh, i just dont fill my drive with stuff i wont use/play etc.

Not strange at all. It's advisable to keep around 10% of your total hdd free like what megagiga said so that your Virtual RAM has enough space to work. I like to keep mine above the 50gig threshold.

Edit - Should say Virtual Memory for clarification

128MB

I have an easily expandable fileserver at home, so whenever i'm running low, i just connect another HDD which means i don't really need to care about free space

15GB on each partition.

About 100GB on each 1TB drive. I learned the hard way.

once time, my game drive dropped to 0 byte

Depending on disk type, I have a couple smaller ones for O/S and temporary stuff, and 2 bigger ones, 1TB one for Steam games and a 300GB one for non-steam stuff, usually F2P.

I usually start cleaning the big one when I drop under 80GB, and the small ones when under 10-15GB...

Around 100 GB free, dunno why xD

Optimally 20% should be left as free space on any boot drive and on the others its pretty irrelevant... More importantly data is automatically written onto the hard drive starting with the outer most track of each platter and then is progressively written further in. As the outer most tracks have a greater capacity than the inner ones due to the nature of the laws of circumference a larger amount of data can be accessed without having to move the arm onto which the laser for read and write operations is mounted which is by far the slowest step. So basically your hdd will get slower the more you have on it in theory, and in practice its not quite so simple because the order in which data is written onto the hdd might be somewhat affected though as a general rule it will still on average get slower with each bit of additional data stored.

when my pc slows down i clean it.

I'm quite tidy now with my hard-drives so I don't really need to think about it but, as a general rule, I'd suggest having at least a 20-30% free space on each drive.
Both mechanical and solid state drives benefit from having free space.

10 GB

Since I use SSD for OS, I tend to start to clear data drives up once there @ about 100GB, these are for 1TB drives, I just don't like it when they're filled with crap, and once it hits 100GB left, that tells me .. I have a lot of crap I am not using, I need to remove :P

What's a HDD...? o.O

8.17 of 175GB.

Around 20GB.

I partition my HDD so it depends on their sizes :V

100GB

When I'm running low ~200 GB I just buy another hard drive.