How much hard drive space should you have on your PC laptop?

That will depend very much on how you use your computer. if you only do office work, web surfing, and listen to a little music, a 40 GB hard drive with Windows Vista, or even a 10 GB hard drive with...

As much as you need to store your files. More than one, less than a terrabyte.

you can use it in an external enclosure that connects to the laptop using either USB, firewire, etc. However, you will never have enough room to install a genuine hard disk inside a decent laptop...

That is impossible to know without having access to your computer. Everybody uses varying amounts of space.

Not unless it doesn't work anymore or you simply want to.