How long does a sobremesa last in Spain?

From twenty minutes to the whole afternoon: it is the time spent at the table after a meal, talking over coffee and sometimes a drink. Sunday family lunches have the longest; at work it is a coffee and back to the office. Getting up as soon as the plates are cleared can look like a lack of interest.

The hour after the meal

what: The time spent at the table after a meal: people go on talking, have a coffee and sometimes a drink. It can last twenty minutes or the whole afternoon.
when: After lunch above all. On Sundays the family lunch is long and the sobremesa longer. At work it is short: a coffee and back to the office.
where: At home and in restaurants alike. In a restaurant the waiter does not bring the bill until you ask for it.
who: Everybody who was at the table. The children get up to play; the grown-ups keep talking.
why: The company is the point. Getting up as soon as the plates are cleared can look like haste, or a lack of interest in the people you ate with.

The sobremesa is the time spent at the table after eating — almost word for word the dictionary definition. English has no single word for it.
On Sundays the family lunch is long and the sobremesa longer; the same old stories get told and people argue about football or politics.
Leaving straight after the meal can seem like a lack of interest in the company. If you must go, say so and apologise, and nobody minds.
De sobremesa also describes things made to sit on a table: un ordenador de sobremesa is a desktop computer.

La sobremesa es el tiempo que se pasa en la mesa después de comer.

The sobremesa is the time spent at the table after a meal.

Los platos están vacíos, pero nadie se levanta.

The plates are empty, but nobody gets up.

Lo siento, me tengo que ir.

Sorry, I have to go.

More about La sobremesa

Sources

The hour after the meal — La sobremesa