Eggs hatch 4-5 days after laying. In the wild, females lay only one egg per milkweed plant, or at most a few. But I often give teachers a leaf or milkweed stem with many eggs attached. This can lead to cannibalism, where the first few caterpillars out eat a sibling still in the egg. Through this and other mishaps, up to 10% of the eggs can go missing. "Mishaps" include eggs falling off, spiders snacking on eggs, and eggs getting lost in dried up leaves.
If you can see a black top to the eggs, this means the egg will hatch within about 12 hours. The black is the large, dark head of the caterpillar inside. Once out, the first thing a caterpillar does is eat its own eggshell. (This behavior might help to explain cannibalism.)
See this amazing video of a monarch hatching, by Dave Hinterberg of Wingra School.
Next, they move out to find a place to dine. Before feeding they shave off the tiny hairs called trichomes that protect the milkweed leaf. As they begin to consume the leaf, they create a crescent-shaped cut in the leaf surrounding the place where they are located. This prevents the plant's sap from reaching the parts of the leaf being eaten. That's a good strategy, because the sap is both toxic and sticky--it can jam the tiny caterpillars mouthparts.
It's common to find these crescent shaped cuts on milkweed leaves, but no caterpillars nearby. That's an indication of how few caterpillars escape predatory insects and spiders to grow to a large size.
For more information
How milkweed plants defend against monarch butterflies ‣ Data Nuggets
Book by Anurag Agrawal: Monarchs and Milkweed.