I’d say around 2-3 years to make a full game. A demo/vertical slice with limited level design/characters/skills etc can be done in couple of months.
You can start from checking this 3 tutorials - “Multiplayer Souls-like action RPG”, “Multiplayer FPS shooter” and “Multiplayer dungeon crawler” DevAddict's Unreal Engine Courses
PS. They cover only gameplay side, so you will still have to figure out how to setup and run servers, handle users, save their data and such sings on your own.