Was bored, so made a simulation to figure it out.

TLDR: 592.2 seconds, or 9 minutes and 52.2 seconds. Very similar to the other comment, so it appears temperature differentials and heat loss to the air are somewhat minor effects compared to the sheer heat mass of the block

Assumptions:

• Copper’s heat conductivity is 400 W/m-K, and specific heat is 0.4 J/g-K, and density is 9000 kg/m^3, and these values do not change over the range of temperatures
• Air’s heat transfer coefficient is 20 W/m^2-K and does not change over the range of temperatures
• The surrounding air does not change in temperature and remains at room temperature (25 C)
• The input wattage is actually 120 W and not just random marketing bullshit
• The copper block’s size is 4 cm x 4 cm x 16 cm (same as other comment)
• The temperature within the copper block differs only by the vertical axis; it is assumed that temperature does not change if you move horizontally into the block

Modeling conditions:

• The block is sliced into 100 equally-sized slices, stacked vertically.
• 120 W is input directly into the bottom slice
• Heat transfer is modeled between each slice
• Heat loss into the air is modeled for each slice (top slice has more heat loss due to more contact with the air)
• Heat changes are calculated per millisecond
• Final time is calculated is the total number of milliseconds it takes for the bottom slice to reach a temperature greater than 100 C