The exact amount of time is not specificed:
He leaped into the lake, would not wait for anyone's
Answer; the heaving water covered him
Over. For hours he sank through the waves;
At last he saw the mud of the bottom
It took Beowulf several hours or perhaps even more to reach Grendel. Deeds such as this one develop Beowulf as mythical hero, emphasizing his great strength, endurance, and resolve.