No simple answer: depends on what you want to do. If you have an undergravel filter, then you'll need an ~8 cm depth, and from my experience that means at least two 25 kg bags of pea gravel for medium sized tanks.
If you're growing plants, you'll be using gravel alongside something rich in nutrients like aquatic soil mixed with silica sand. So the gravel is just used at the top for decorative purposes, maybe 1-2 cm on top of at least 4-5 cm of soil/sand mix. So a single 25 kg bag would be enough.
If all you want is to hide the glass at the bottom of the tank, and you're decorating with rocks and bogwood (or plants that don't need substrate, like Java fern and Anubias) then you don't need much gravel at all. A 25 kg bag will be ample.
Cheers, Neale