A lot depends on how you read and which books you're counting. The core novel, 'It Ends With Us', is about 115,000 words—that's a solid weekend read if you're absorbed, maybe 6-7 hours for an average reader? But then there's 'It Starts With Us', the sequel, which is shorter, maybe around 90-000 words. For the whole 'series' as it stands, you're looking at roughly a 200k-word commitment, not counting Colleen Hoover's little bonus chapters or letters she sometimes adds online.
Reading speed varies wildly, though. I tore through the first one in a single, very late night because I couldn't put it down—the chapters are short and the dialogue pulls you along. My friend, who only reads on her commute, took two weeks. If you're listening to the audiobooks, both combined run about 20 hours. The sequel feels quicker, maybe because you already know the characters, but the emotional weight can make you need to pause.
Honestly, the time isn't just about page count. These books make you stop and think, or just stare at the ceiling for a bit after certain scenes. That processing time is part of the read, for me at least.