The Brief: For days means "in abundance".

Bluesquiggle
DEEP DIVE

For days is an idiomatic expression used to describe a conspicuous abundance of a certain trait, feature, or item. The idea is that someone posesses so much of something that it seems to last “for days.”

The phrase is a synonym for all the.

Legs for days

A common usage is “legs for days.” This particular iteration typically describes a woman with long, attractive legs.