I live in the US where the standard of a full time job is a pretty rigid 40 hours per week/8 hours per day. Business hours are 8am-5pm. I understand this varies country to country, but where did this standard come from?
From the idea first formulated by utopian socialists (such as Robert Owen) and later promoted by increasingly strong trade unions in late 19th- and early 20th-century industrialized countries that a tolerable, balanced life ought to be comprised of equal parts toil, leisure and sleep — and 24 divided by 3 is 8.
"8 HOURS LABOR, 8 HOURS RECREATION, 8 HOURS REST" and variations thereof were popular rallying cries, and the culmination of decades of struggle against overwork and in support of shorter hours — which in turn were additionally hailed as a solution to unemployment. By the '20s, most industrialized countries had introduced 8-hour-workday legislation.
One might argue that the real question is: why didn't working hours keep decreasing? More radical unions (such as the IWW) argued for a 4-hour workday, 4-day workweek as early as the '30s. Around the same time, Keynes famously prophetized that working hours would be below 15 a week by the turn of the 20th century. So, what gives?
Many factors were involved, including of course hostility from the bourgeoisie. But maybe the single most important one was the gradual socio-political integration of unions, most of which lost their revolutionary streak as a result. Organized labor had become a more-or-less cooperative partner of capital, more so than its indomitable antagonist.
This was particularly true after WWII, when a compromise between labor and capital designed to ensure post-war prosperity and stability was struck. The Cold War also meant that labor radicalism came under increased suspicion and was easily denounced as communist subversion, and "reasonable" union reps sought to distance themselves from "red" agitators.
By the time the Soviet Union collapsed, political trends (neo-liberalism) and economic transformations (post-Fordism) had significantly weakened unions and their ability to both formulate demands and organize workers. We could also discuss how capitalism managed to absorb a longing for shorter hours with the gig economy, but that's another subject entirely!