yes if it just idle
I have try it, with battery saver mode, no background task, no data connection, just use for receive text and call
3 hours idle consume just 1% battery. So if it calculate it will standby for more or less 12,5 days.
Definitely much better than Android side of things even with any Android battery saving type features.
FYI, the Nokia 720s are for my aging parents who had Nokia 3310 and 6610 phones which lasted a good 4-7+ days. They don't use much internet/wifi at all just needed a new phone and Android wasn't for them.