The battery is usually in the charger and therefore ages quickly and can no longer be used for phone calls for more than 10 minutes.

If used, 2 days `!

We only use the phone as a reserve and rarely use it. When switched off, there is hardly any self-discharge even over several months. Switched on, it certainly lasts for several days.

there's no general answer to this because it depends on the usage: the more whatsapp and/or (slow!) surfing, the more battery consumption.
for "normal" use (some whatsapp, no surfing, a few short phone calls) I would say: at least 1-2 days.