It opens on the Thames, not in Africa: a yawl at anchor waiting for the tide, and a sailor named Marlow talking to four men on deck. He tells them how he took command of a river steamboat for a trading company and went upcountry, past chain gangs and a shaded grove where worn-out workers had been left to die, into stations where nobody speaks of anything but ivory, which travels the other way, downriver, in a thin trickle. At the far end is Kurtz, the company's best agent, who went out to improve people and stayed to be worshipped by them. Conrad never names the country he is describing, or its river. Under two hours, but not an easy two hours; sentences will get away from you.
Heart of Darkness is 38,527 words. At an average adult reading pace of 300 words a minute that is about 2 hr 8 min, or roughly 1 hr 36 min if you read quickly and 3 hr 13 min at a more deliberate 200 words a minute.
That count is measured from the complete public-domain text rather than estimated from a page count, so you can compare all the books side by side.