The recommended time step size Δt for a transient thermal analysis is related to element conduction length and material properties.
The larger the thermal diffusivity, the smaller the Δt and element length should be. The recommended time step is given as:
Δt ≤ δ2 ∕ 4α
α = thermal diffusivity = K / ρc (Units are length2 / time)
δ = conducting length (for line elements this is the element's length, for plane or volume elements, it is the diagonal of the inscribed circle or sphere)
K = conductivity
ρ = mass density
c = specific heat