A flashdrive of 16 GB having data of about 8GB how much time normally take to quick format the 8GB data from USB 2.0?

If you are executing a FORMAT instruction, the time it takes has nothing to do with the data content of the device. It can be full 16 Gb, empty or half ful, as in your case with about 8 Gb.

FAST FORMAT is fast, takes just seconds, the drive is not checked for bad sectors AND actually nothing is erased but pointers of the folders.. Anyone looking at the hard drive or storage device would not see any data and assume the drive is erased. The files are actually still there and using some procedures, the volume could be re-built, to gain access to the most of those files again.

A Full Format runs an additional step that checks the drive for any bad sectors. This check is what makes the full format take so much longer than a quick format but just like the quick format the files still exist and the volume could be re-built to gain access to any of the files on the drive again.

The fact that device might have more or less files, has noting to do with this process