Where Epic keeps the number
Fortnite is handled separately from the rest of the Epic library. Epic's own support documentation covers where to find time spent playing Fortnite, and the route has changed more than once - historically through account settings and player-data tools rather than the launcher's library view.
Because Epic has moved this around, the reliable instruction is: start from Epic's Fortnite support pages for the current location rather than from a video that may be two UI revisions out of date.
What in-game stats do and do not cover
Fortnite's career tab shows statistics for the current season and mode - matches played, wins, eliminations, and similar. It resets with each season, and it is per mode, so Battle Royale, Zero Build, Creative, Save the World, and the newer modes fragment the picture further.
For a game that has been running since 2017 across dozens of seasons, seasonal stats are close to useless for the lifetime question.
Creative and custom modes complicate it
A large share of modern Fortnite play happens in Creative islands and user-made modes. Time there does not appear in Battle Royale statistics at all, and individual islands keep their own scoreboards if anything. A player who mainly plays custom modes could look barely active by the career tab.
Measure the client instead
PlayCounter times the Fortnite client on Windows. Every mode counts, seasons do not reset it, and lobby and queue time is included - which is a substantial part of a Fortnite session that no match-based statistic captures.
It works the same whether you launch from the Epic Games Launcher or a shortcut, and it puts Fortnite in one list with the rest of your games so you can see the real proportion. Everything stays on your PC, and no Epic account access is involved.
As with every game it tracks, the record starts from installation. It will not recover the seasons you already played.