It usually opens 3 hours before departure at peak times, basically early in the morning when they have to fill an A321. After morning rush I'd say around 2.30 hours before but that may vary.
Linate is very small and close to the city, so I wouldn't go there too early, whilst the newly renovated Malpensa require the passengers to walk a lot (way too much and I like walking!) so keep that in mind as well.