= Solution
Linear Curie behavior fails gradually when the Zeeman <energy> of a substantial moment is comparable to thermal <energy>. Its appropriate scale is
$$
\boxed{g\mu_BJ B\sim k_BT,\qquad
B_{\rm nl}\sim\frac{k_BT}{g\mu_BJ}.}
$$
At $T=300\,\mathrm K$, $k_BT/\mu_B=446.6\,\mathrm T$. Thus $B_{\rm nl}\sim446.6/(gJ)\,\mathrm T$: about $223\,\mathrm T$ for $g=2,J=1$, or $447\,\mathrm T$ for a spin-one-half moment with $g=2$. Larger moments saturate at smaller fields. The source does not specify numerical $g,J$, so it determines this parametric scale rather than one universal room-temperature threshold. At sufficiently large field the moment tends toward $g\mu_BJ$ and cannot keep increasing linearly with $B$.
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