You cannot tell. What is normal usage? For some, excessive gaming is normal. For others, taking 200 photos a day is normal. Or streaming their favorite shows via 4G/5G while on a train or bus is normal for many with others barely leaving WiFi covered areas.
Some use it in bright sunlight (screen brightness up high), some indoors only.
Some make 20 phone calls in areas with bad reception, others make barely one call or only via WiFi.
With the Pro Max, going from 100 to 20 anything between 5 hours and 2.5 days is possible in my opinion, depending on what the user considers as „normal usage“.