How much can you actually pack in an underseat bag for a weekend?

Across the reviewers we read, a 14–18 inch underseater comfortably holds 2–3 days of clothing, toiletries, a laptop, and chargers. r/HerOneBag posters using the Travelpro Maxlite 5 report multi-week trips are possible with careful packing cubes, while Amazon Basics owners describe it as best suited for overnights and short hops.