The way I see it, compared to my regular workout (drive to the gym, spend about 2 hours there, drive home, usually 2-3 hours total), p90x vids are super short in comparison. Usually around 60 minutes total, and 15-20 min is warm up/cool down. And, of course, walking into my basement doesn't take as long as driving to a gym.
The 40 minutes of actual workout is just death if you're actually going hard and keeping up with the video without pausing. I get a waaaaaaaaaay better workout out of p90 compared to the gym, where I'm (unintentionally) taking way too long breaks between sets, getting too many water breaks, walking around, talking to people, checking my phone... list goes on. It's so easy to get distracted, and after hitting a plateau from regular gym workouts, I lost like 30 pounds after 40 days of p90x2. It feels really good to know that if I just get through the 40 minutes of hell, I will see results. So the length doesn't bother me at all.
As for the monotony of the videos... it's true, but at the same time I feel like working out in general is pretty monotonous. It's a lot of the same moves again and again and again. I'm ok with the repetition, I get joy out of seeing how I can improve compared to my last workout, or modify the move to make it harder than last time. When it comes to p90x, I'm still nowhere near Tony Horton doing like 30 fucking chinups after 30 minutes of... non stop chinups. I'm keeping like MAYBE 50% pace with the guys and gals in the video, and still getting crazy results. So, as repetitive as the vids may be, there's also tons of room for improvement, so that keeps me going. I also don't plan to stop after 90 days.
At the same time, I made the jump from P90x to P90x2 just to mix things up. I did a couple of Insanity vids years ago and they were pretty fun too from what I remember!