If the MacBook battery drains fully in 10 hours, how long will the battery last in years?

Battery health decline is not linear. Typically, I see Mac notebook batteries drop from 100% health (more or less) to the low 90’s over the first 100-200 cycles then hold there for several hundred more cycles fluctuating around 90%. Then they begin to decline again until their eventual demise.

The lifespan of lithium battery is measured a not only in charge cycles, but also in objective time. A 7-year-old battery is likely to be failing, even if it has relatively few cycles on it.

chdsl wrote:

If the MacBook battery drains fully in 10 hours, how long will the battery last in years?

That's not the right metric to predict battery lifetime. Batteries are highly variable in their lifetime and over time they won't last as long. We have a 2013 MacBook Air whose battery failed and was replaced in 2021. Now, in 2025, the "new" one still shows 100% capacity. We also have a 2010 MacBook Air whose battery is still the original part that came new with the laptop. It has shown 83% capacity for more than the past 6 years.

But, my iPhone 11 gets plugged in everyday and to 100% and by ~20%, I plug it in. In Aug 2023, its battery health was 86%, and now 81%, but that 81% is there for more than 7 months.

Batteries are managed very differently on iPhones.

I also have a 2019 MacBook Pro whose battery showed 91% capacity after 30 months. After 36 months it showed 93% capacity. Now after 76 months, it shows 86% capacity. How long will it last? No one knows for sure, I will use it until Apple says "replace battery" and then I will replace it if that is still an option, otherwise I will replace the laptop (might want to do that anyway as new ones are much faster).

But I can't imagine having the emotional energy to worry much about battery lifetime over the past 15 years for any of these devices. I simply use the built in battery health management and figure that will have the best outcome (the older MacBook Airs can't even use BHM).

The rate of decline in battery health is not linear. My observation is that, it slows as the capacity goes down. However, that's just one person's observation. Take it for what it's worth. The battery health shown on the device is also an estimate. It sometimes changes significantly after a major update because it gets recalibrated.

neuroanatomist gave you the right answer: It will last as long as it lasts. There's really no way to predict the end point with any great degree of accuracy.

If your phone's battery health is at 81%, I'd consider having Apple run diagnostics. It may be time to replace it.

The short answer is that it will last as long as it lasts. Enable the option to manage battery health and don't worry about it.

About battery health management in Mac laptops - Apple Support

The best practice is to plug into power whenever it’s convenient and run on battery only when you cannot connect to power. Your Mac uses Optimized Battery Charging, which is enabled by default. That is designed to charge the battery in a way that balances increased battery lifespan with your personal usage patterns.

The longer answer is that atteries are a wear item like tires on a car. Their useful life is finite, and they will decline over time. Battery 'health' is the ratio of the current maximum capacity of the battery to the design maximum capacity. Apple considers a value of ≥80% to be 'healthy'. 

Many things can affect the rate of decline, such as charging behavior and most significantly temperature. For example, if you leave the Mac (especially if the battery is fully charged) in a hot car for a few hours, that can damage the battery and shorten its lifespan significantly. The car tire analogy holds, if you take fast corners and drive on rough roads, you'll need new tires sooner.

Stop trying to micromanage the battery. When convenient to do so plug the computer into a power source. When a power source is not available use the computer on battery. The MacOS manages the power/charging system very well.

chdsl wrote:

The battery was manufactured in July 2022 by SMP, and had never been used.

Indeed, but that’s three years ago. The point remains that it will last as long as it does. Note that there’s a stochastic part to this – the same way your new battery is 7% over the design capacity (I’ve had new Macs range from 97% to 112%), the rate of decline also varies intrinsically from one battery to the next.

Its real battery health capacity is 83.2%, not 81% as Apple states. And, its cycle count is 1177, much above Apple's 1000. The battery health capacity had been like that for +7 months.

All of which demonstrates that there is no way to accurately assess the lifespan of a battery at that level of granularity. At best these numbers are broad outlines, and that's the best you get.

chdsl wrote:

The battery was manufactured in July 2022 by SMP, and had never been used.

Sitting without being used isn't great for lithium ion batteries.