Apple’s expectation/guarantee is the battery
should be able to retain 80% up to 1000 cycles — it has doubled in the 15 series. The numbers indicate you’re needing to fully recharge the battery at least once per day, on average, which probably falls under the ‘heavy user” column. The shorter answer is the battery should last two years before hitting the recommended replacement threshold.
The longer answer:
242 cycles / 180 days (since 15 series launch) = ~1.34 cycles per day
1000 cycles / 1.34 cycles per day = ~746 days
And if we do a linear projection via percentage loss
100 percent - 80 percent (target) = 20 percent, 5 percent (loss) / 20 percent (target) = 1/4
1/4 * 4 = 1 (whole), then multiply the other side by the same, 180 days * 4 = 720 days
However, battery degradation is not linear; the capacity will drop a little quicker at first, the endurance will level off (for the most part), then plummet.
Not iPhone test results but here’s a fair representation of capacity to usage: