I think most likely the Tablet part of the Nintendo Switch will have similar battery life to gaming tablets like the Nvidia Shield K1 tablet because the Switch uses a Nvidia Tegra processor and video chip in the tablet.
The Nvidia Shield K1 tablet gets about 5 hours of battery life when gaming according to
TechRadar.
The Switch's battery life maybe slightly better because the Switch may use a smaller non-HD or 720P screen with a backlight which is not as bright as tablets like the Nvidia Shield Tablet, so it uses less power than a 8 inch 1080P screen found on the Nvidia Shield tablet, and the Switch may use less powerful speakers, and have less features like Bluetooth, GPS, HD webcams, motion-tracking, light sensor, MicroSD card slot, etc which use up more battery power. The Switches battery maybe bigger, so users can play games longer.
I think the battery in the Switch's tablet won't be easy to replace like most tablets, but the tablet's battery would last for many years like batteries in higher quality tablets like the iPad 2 where the battery and tablet still works after many years of continuous use. User's would most likely need to hire a console repair company to replace the battery if it breaks.
Although, Nintendo may make the Switch's tablet battery replaceable like smartphones and some older tablets and laptops with removable batteries.
The battery life of the controller would probably last many hours if it does not have vibration feedback, or a speaker like the Wiimote. I think it's battery life would be over 30 hours like the wiimote depending on what type and capacity the batteries are on the Switch's controllers.