A missed watering or two is rarely fatal on its own. Most established plants have some buffer built in, whether that's moisture reserves in thicker leaves and stems, a deep root system reaching moisture lower in the soil, or simply being naturally drought-tolerant. The plants most at risk from a short lapse are shallow-rooted seedlings, anything in a small pot, and naturally thirsty leafy vegetables like lettuce and spinach, since they have the least reserve to draw on and can wilt within a day or two in hot weather.
Wilting itself isn't necessarily a bad sign, it's usually the plant's own defence mechanism, reducing the leaf surface exposed to the sun to cut water loss while the roots are under stress. The genuinely useful test is what happens after watering: if a wilted plant visibly perks up within a few hours of a good, deep watering, it's almost always going to be fine. If it's still limp and hasn't responded a day later, that's the point to start suspecting more serious root damage rather than simple thirst.
if a wilted plant visibly perks up within a few hours of a good, deep watering, it's almost always going to be fine
Leaves that have gone crisp, brown and papery, rather than just soft and drooping, are a different situation: that tissue is actually dead and won't recover no matter how much water follows. It's still worth watering the plant properly rather than giving up on it, though, since a plant can often push out fresh new growth from undamaged stems and roots even after losing a significant portion of its leaves.
When rehydrating a badly dried-out potted plant, resist the urge to just tip a full watering can on top and walk away. Extremely dry potting mix can actually repel water rather than absorb it, causing the water to run straight down the inside edge of the pot and out the drainage holes without properly wetting the root ball. Standing the pot in a tray or bucket of water for 15-30 minutes, so it can soak up moisture from the bottom, rewets dried-out mix far more reliably than top-watering alone.
For an in-ground plant that's gone through a longer dry stretch, water deeply and slowly rather than a quick surface splash, so moisture actually reaches the root zone instead of just wetting the top few centimetres and running off. Avoid fertilising a stressed, recently dried-out plant straight away; a plant with damaged roots is less able to take up nutrients properly, and a dose of fertiliser on top of drought stress is more likely to cause fertiliser burn than to help it recover. Give it a week or two of consistent watering to recover first.
As a rough guide by category: succulent-type and Mediterranean herbs like rosemary, thyme and lavender are genuinely forgiving and can go a week or more without trouble; most trees, shrubs and established perennials have real buffer once their roots are settled in; but seedlings, leafy vegetables and anything in a small container have very little margin and are the first to check on after a lapse.