I've tried all modes, 1, 3, 7 , 35. Secure Deletion is working with every type of file and naturally it takes a while to overwrite the file, but I noticed that video files weighing e.g 3gb were deleted instantly, which doesn't make sense. I used WinRAR and TreeSize to go to the recycle.bin and saw that the file was there and it wasn't deleted at all.
I've reinstalled it and I have the newest version (free). If I disable Secure Deletion and clean the trash with 'Normal file deletion' the files are indeed deleted, which confirms to me that there is something buggy in Secure Deletion.
Again, all other files even larger than 3gb are normally deleted, minus video files. I used 3gb as example because that's what I have, I don't know from how big it stops working.
It's also good to mention that if I use TreeSize I can go into the recycle.bin and get the file back and it's not modified at all, which shows that CCleaner didn't even overwrite the data.