Depending on the battery you use, it's between 40h to 60h playtime.
I successfully used the 2200mAh batteries from "thepartguru" on iPod classics with iFlash Quad. The advantage over the 3000mah ones is, that it still fits with a thin backplate.
You should also be aware, that putting more than 256GB into the device is considered as "unstable" depending on the content you use. Having more than 50000 tracks may lead to unexpected behaviour (random reboots, showing the wrong covers for items, etc.). Technically iFlash Quad supports up to 4TB microSD space, but in practise it just does not make any sense. I would not use more than 512GB - my most stable experience was 256GB.
The reboot loops are extra nasty, because once they happen, they do at random days of a week and if you don't use your iPod every day it fries your 2200mah battery after a few days of reboot loops... expensive experience ;)
I successfully used the 2200mAh batteries from "thepartguru" on iPod classics with iFlash Quad. The advantage over the 3000mah ones is, that it still fits with a thin backplate.
You should also be aware, that putting more than 256GB into the device is considered as "unstable" depending on the content you use. Having more than 50000 tracks may lead to unexpected behaviour (random reboots, showing the wrong covers for items, etc.). Technically iFlash Quad supports up to 4TB microSD space, but in practise it just does not make any sense. I would not use more than 512GB - my most stable experience was 256GB.
The reboot loops are extra nasty, because once they happen, they do at random days of a week and if you don't use your iPod every day it fries your 2200mah battery after a few days of reboot loops... expensive experience ;)