Sheen levels are subjective to a degree as different paint manufacturers have different interpretations of how much reflectivity a matt, satin and gloss paint should have. In general, matt paints produce a finish that has no or very little sheen. These paints are usually used on walls and ceilings and are less likely to show any surface imperfections.
Another way to describe satin is eggshell. In other words, the sheen level is about the same as the surface of an egg when 1 coat has been applied. 2 coats will very slightly increase the sheen level.
Gloss finishes have a higher reflectivity than satin or satin-gloss paints. Some paints and varnishes are even labelled as super-gloss. Typically, you will be able to see a good reflection of objects in a gloss paint. Gloss paints are more commonly used for home fixtures such as skirting boards, architraves, doors and so on.
Chalk paint is a popular product for up-cycling interior furniture. It offers a little protection for surfaces; however it is more of an aesthetic finish. Wax is the most common surface sealer for chalk paints, but you can also use a varnish, taking care not to overwork the varnish, as this can pull the paint. Varnishing over chalk paint will typically 'warm' the colour. E.g. shades of white become creamier.
This is a difficult question to answer as it can depend on the type of substrate, the preparation of the substrate, the application and type of paint and the amount of weathering that the surface is subjected to. As an example, paint on a cliff top property that is subjected to high winds and sea air will wear more quickly than paint on a sheltered town house. Some manufacturers supply paints that are advertised lasting up to a specified number of years as long as the correct preparation and application conditions are met.