I have had Facebook Messenger off my phone for a just about a week now and I think it may have actually made a tiny difference. These things are actually impossible to tell. That app usually never did that much damage. It was the facebook app itself that was always shitty with battery.
I'm doing a little experiment for the next week. I'm leaving Stamina Mode on at all times in order to turn off data/Wi-Fi when the screen is off. Since I'm at work most of the day, I don't really check my phone much until maybe around lunch time. I have left the option off to decrease the CPU performance because I noticed when I do have it checked on, while the phone is very fast, I think I do notice a lower overall frame rate. Under these conditions yesterday, I believe that I ended the night at 73% and when I woke up this morning, the phone showed 72%. I had 1:20 of screen time yesterday while browsing and listening to music. I was also listening while the screen was off, did some downloading, and talked on the phone for 20 mins.
I'm not going to run my phone down to 10% anymore as its harmful to the battery, but I'm assuming that it will be around 40%, which will mean in general for me - I would get 3 days (72 hours) with light/medium usage.
Also, I'm not going to need to open up the flaps to charge anymore after my magnetic charging adapter comes in tomorrow!