Question about Lenovo ThinkBook 14 Gen 8

Unfortunately, we can't give you a definitive answer, as actual battery life will depend very much on how you use the device, the settings you select, the applications you use, etc.
For classic office use (e-mail, word processing, etc), ~8-10h are conceivable, but if you switch to something heavier, a video game for example, it's likely that battery life will be less than ~5h.