How long dose it take to beat doom eternal on easy mode?

Like most people... I want to play doom eternal on easy mode. But how hard and long will it take me?

Well, it depends on your playstyle... it has about 13 missions which are HUGE. One Doom Eternal mission/map is about the size of 2 Doom 2016s missions.

During my first run I stayed about 1,5 - 2 hours on each map (looked for every secret, played slayer gates, did all challenges), except Sentinel Prime, which is quite short (except for the reading parts :-) ), so, if you don't rush, expect the campaign to take 16-22 hours of your time.

If you just do the main objectives and ignore all secrets and so on, you might be faster. But don't expect less than 12 hours for your first run. There's much to learn and it takes a while to get used to the basic gameplay mechanics (e.g. jumping puzzles and combat loop).

...Play it and find out. ;)

I played on HMP and I didn't rush, probably took me about 15-20 hours.

Really depends on how well you know the game and your skill level. On my first play through on Hurt Me Plenty it took I think at least 20 hours. I improved my time on Ultra Violence, and by the time I played on Nightmare I knew it well enough to finish in under 6 and a half hours, and that's with all Slayer Gates and Secret Encounters,  and 100% on combat, collectibles, and exploration.