One litre is exactly 1,000 millilitres. It is the plainest conversion in the volume category and the one most people do without noticing, but the litre's own definition is more interesting than it looks.
About the units
The litre is not an SI unit. The SI unit of volume is the cubic metre, and the litre is defined as exactly one cubic decimetre — a cube ten centimetres on a side — which makes it exactly 0.001 m³. That definition has been stable since 1964, but between 1901 and 1964 the litre was defined instead as the volume of one kilogram of pure water at maximum density, which turned out to be 1.000028 dm³. Measurements published in litres during those decades carry that tiny discrepancy. The millilitre is a thousandth of a litre and therefore exactly one cubic centimetre, which is why ml and cc are used interchangeably in medicine and engineering.
The exact factor
The factor is exactly 1,000: shift the decimal three places. 0.75 l is 750 ml; 250 ml is 0.25 l. The equivalence worth carrying alongside is that 1 ml = 1 cm³ exactly, so a volume given in cubic centimetres needs no conversion at all to millilitres. That identity is what makes the metric system convenient for anything involving both volume and dimensions: a container 10 by 10 by 25 cm holds 2,500 cm³, which is 2,500 ml, which is 2.5 litres, with no arithmetic beyond the multiplication.
Where you meet this conversion
Everywhere. Beverage containers, medicine doses, engine oil, cooking, laboratory work and fuel. In medicine the millilitre has entirely replaced the cubic centimetre in written orders, because cc handwritten can be misread as zeros. Engine displacement is quoted in litres for cars and cubic centimetres for motorcycles, so a 125 cc engine is 0.125 litres and a 1,600 cc engine is the 1.6 litres a car listing would state.