It will tell you the exact dilution rate on the container. If the glyphosphate is not in its original container or if the label is illegible then dispose of it at the Council recycling centre and buy some new. It is not expensive.

1 glug per 2 litres ;-)

From memory, I used to use 300ml in a 15 litre sprayer

It will depend what you're using it on. Woody perennials need a stronger concentration that soft annuals. As Bulbaholic says if there is no label on the bottle/packet then you shouldn't be using it.

Rather than throwing it away, err on the strong side and use it up. To dispose of it is a real pain as wherever you take it, be it council recycling or other, they have to abide by regulations and treat it as toxic waste. So do everyone a favour and use it as it's intended and save someone all the hassle of having to spend money and energy disposing of it the correct way.

But please, PLEASE don't throw it down the drains.

Well the dilution rate rather depends on which strength glyphosate you've got there - some are stronger than others.