I’m seeing monitors offering USB-C connections to simultaneously drive the display and to power the laptop.
Question:
Does USB-C provide sufficient current to keep a laptop adequately charged when using it for 8+ hours at a time?
Yes and no.
USB-PD (Power Delivery over USB-C cables and connectors) has its limits. All power sources (Monitors, Wall-worts, hubs, and docks) have their limits and not all support the USB-IF's maximum PD levels. Also some cables don't support the max current levels. Also, any hub, etc, in the chain will draw some power for itself an other connected devices.
If, and only if, every piece in the chain (cables, power supply, hubs, ...) support a wattage equal to or higher than the notebook or tablet plus all other connected devices require will you then be able to have the notebook/tablet run totally from the USB-PD power and not drain the battery.
To accurately answer the OP's question, we need to know what is the exact power requirement of the notebook, the power that is supplied by the monitor, and the power rating for the cable.