Battery on the iPhone XS is generally just under 10 hours. If you tend to use your phone more for audio playback and phone calls than internet browsing you can expect more out of the battery. Keep in mind that the more recent iOS versions tend to use more power as well, but the iPhone XS holds up.
When you purchase a refurbished iPhone the battery has likely been slightly depleted but at Frank We guarantee our batteries retain a minimum of 80% of their original capacity (which Apple has declared sufficient for efficient battery performance).
Apple's lithium-ion batteries are designed to hold at least 80% of their original capacity for a high number of charge cycles, this will vary depending on the product.