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We generally tend to underestimate sizes and masses of celestial bodies. A little giveaway is that for all non-astronomical means and purposes we consider the earth's mass infinite without any measurable error.³

Let's make an estimation: How does the heat stored in the planet Earth relate to humanity's energy consumption? I'm only interested in an order of magnitude here. Let's assume that the average specific heat of the earth's matter is that of silica (SO₂), ca. 0.7 J/(g*K). This leads to the following results:¹

Specific heat of silica (J/(kg*K))              7.00E+2
Earth's mass (kg)                               5.97E+24(2)
Earth's energy/K, assuming it's all silica      4.18E+27
    
World primary energy supply 2015 (Mtoe)         1.36E+4
J/Mtoe                                          4.19E+16
World primary energy supply 2015 (J)            5,60E+20
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Years of world energy supply from ΔT=1K         7.31E+06
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That's actually less than I thought, by a factor of 100 or 1000 or so, but still ... long.

_{¹ The original primary energy consumption number is from the IAEA. Mtoe stands for mega ton oil equivalent, roughly 4,187e+10 J.}

_{⁽²⁾ Give or take 10^20}

_{³ Obligatory (but somewhat depressing) xkcd.}

_{It's entirely possible that I made a mistake and the result is off by a few decimal digits (although it's probably not too small); I appreciate corrections.}