If your walls are smooth, estimate on covering 8 square metres with a litre of matt or semi-gloss paint. Divide your area figure by that amount to arrive at the number of litres you need for a first coat. If you are using finishing paint on porous, rough or previously unpainted walls, a litre will cover considerably less than that.
You can count on more coverage for the second coat. Always check the coverage information on the label of the paint can, as different types of paint have different spreading capacities. Five litres of paint is generally ample for a 4m x 5m room with a 3 metre ceiling and smooth, previously painted walls. The ceiling of the 4m x 5m room would take 1.5 litres (if you must buy 2 litres, you can reserve the extra for repairs). Professional painters often allow for one quarter as much trim paint as wall paint. You can make a more accurate and economical purchase if you calculate the trim area as suggested above. How long will it take? Estimating time brings in a new set of variables. In one hour you are likely to cover about 7 square metres of an ordinary surface, or about 5 square metres of bare wood or plaster.
You will probably be able to paint that 4m x 5m room with one coat in five to six hours. If you apply two coats, you must allow time for the first coat to dry. Allow four hours for water-based paint and overnight for oil-based paint (check the paint can label). You can safely reckon on two or three days to paint a room and clean up afterwards.