Most adult ski helmets fall between 350–500 grams. If you're shopping at the lighter end—under 350g—you'll notice less neck strain during long days on the mountain. Helmets over 500g can start to feel fatiguing after hours of riding, especially if you're making lots of turns or hitting the park.
That said, don't chase the lightest helmet at the expense of safety. Features like MIPS (a rotational impact protection system) and adjustable ventilation add a bit of weight but are worth it for the protection and comfort they provide. A difference of 50–100g might not sound like much, but you'll feel it over a full day.
Also keep in mind that fit matters more than the number on the scale. A slightly heavier helmet that fits your head properly will feel lighter and more comfortable than a super-light helmet that shifts around or pressures your skull. Focus on a snug, even fit first, then compare weights among helmets that fit you well.