I'm seeing a blank screen with cursor(underscore) at the top left. My laptop is accessing the hard disk continously. It's been around 15 minutes.
What command did you use? Just for the record, if you used dd without specifying a block size, it will take an eternity. If you specify a decent block size, it goes quicker.
That said, it depends on so many factors that it's impossible to give you a decent answer. (For example: Disk speed, interface type, whether there are errors on the disk, ...)
My first question is: why do you want to zero it out? If it's for your own use, just repartition. If you want to trash the disk, the most efficient way is to use a nice big hammer on the disk until its heavily damaged. That should stop most amateurs getting any data from it.
The last time I did this before selling a disc a couple of years ago, dd-ing with a 4086 byte buffer and appropriate numbers for the size of the disk, I was getting 2-3 GB/minute (there's a dd option for getting occasional progress reports, though I forget the details). So a 500GB disc would take around 200 minutes or 3 and a bit hours.
Remember, you've got to generate 500GB of data (that it's all "0" doesn't matter), then write it to the disc. If you're on a USB-2 connector, for example, you'll be limited to 480MB/sec, and 500GB @ 0.48GB/sec is a thousand seconds (18 minutes) even with nothing else happening at all.