Centuries. Humans are reproducing much faster than Coronavirus is killing. 360k babies are produced a day. Coronavirus death toll is 81k to date. Nevertheless, they'll find a vaccine in a year's time and eradicate it.
Thanos is at the top of a pyramid fighting swarms of Xenos that are climbing up to kill him. I'm guesstimating there are 16,000 aliens, but this is not 100% verified. You can use your own estimate based on this scene from Alien vs. Predator if you want.
Round 1: Thanos fights the Xenos hand to hand.
Round 2: Thanos fights the Xenos with his double-edged sword.
Round 3: Thanos fights the Xenos with the Infinity Gauntlet holding the power stone.