Was intrigued, 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 - it appears temperature differentials and heat loss to the air have opposite effects on thermal throttle time and mostly cancel themselves out. For the most part, heat transfer and heat loss appear to affect the thermal throttle time less than the sheer heat mass of the block by several multiples

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.
• Each slice starts off with a temperature of 25 C
• 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)
• Temperature changes are calculated per millisecond
• Final time is calculated by the total number of milliseconds it takes for the bottom slice to reach a temperature greater than 100 C

Fun facts I found from playing around with the model:

• According to this model, at the time that the CPU thermal throttles, the top of the block should be 85 C
• If we assume instantaneous heat transfer, time to thermal throttle goes up to 703 seconds (11 minutes and 43 seconds). Difference is about 2 minutes.
• If we assume no heat loss to the air, time to thermal throttle goes down to 500.0 seconds (8 minutes and 20 seconds). Difference is about 1.5 minutes.
• The copper block should be able to prevent throttling as long as the CPU remains idle (30W for AMD CPU's). The CPU should cap out at around 82-83 C.
• The copper block can prevent thermal throttling for a 170 W CPU for 368.1 seconds, or 6 minutes and 8.1 seconds