It is pretty much impossible to estimate with the information your provide, plus it would require somebody with detailed technical knowledge of the battery and its testing in development (nobody here works for Apple). It really depends upon your model phone and how much time you plan on doing all these activities. You say you plan on watching video, which uses a lot of power, but is that for 5 minutes per month or 12 hours per day? No, no, even if you answer that I doubt anybody here would tell you how long it would last.
To learn about maximizing (and the difference between) battery life and lifespan, read this support document --> Batteries - Maximizing Performance - Apple
Read this support article about iPhone battery and performance to learn about the battery health feature and what is considered normal performance --> iPhone Battery and Performance - Apple Support
To see the feature go to: Settings > Battery > Battery Health
"A normal battery is designed to retain up to 80% of its original capacity at 500 complete charge cycles when operating under normal conditions. The one-year warranty includes service coverage for a defective battery. If it is out of warranty, Apple offers battery service for a charge." A degraded battery may display the notice: "Your battery’s health is significantly degraded. An Apple Authorized Service Provider can replace the battery to restore full performance and capacity." If it is not at or above 80% "normal", consider contacting Apple about a battery replacement. --> iPhone Battery Replacement - Apple Support
According to the above quoted information, you could be losing almost 2% per month the entire first year before Apple would deem this to merit a warranty replacement.
- Requires iOS 11.3 and for iPhone 6 and newer iPhones only, not iPad or Watch.
Also read this post by ASC contributor Lawrence Finch which has a very user-friendly explanation of battery features --> Bettery health maximum capacity - Apple Community
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Edit: I see somebody else said 2-3 years. My battery is 7 years old and still has 83% health. See how difficult it is to give you an estimation?
