  New properties can arise at van der Waals (vdW) interfaces hosting a moir\'e
pattern generated by interlayer twist and strain. However, achieving precise
control of interlayer twist/strain remains an ongoing challenge in vdW
heterostructure assembly, and even subtle variation in these structural
parameters can create significant changes in the moir\'e period and emergent
properties. Characterizing the rate of interlayer twist/strain relaxation
during thermal annealing is critical to establish a thermal budget for vdW
heterostructure construction and may provide a route to improve the homogeneity
of the interface or to control its final state. Here, we characterize the
spatial and temporal dependence of interfacial twist and strain relaxation in
marginally-twisted hBN/hBN interfaces heated under conditions relevant to vdW
heterostructure assembly and typical sample annealing. We find that the
ferroelectric hBN/hBN moir\'e relaxes minimally during annealing in air at
typical assembly temperatures of 170{\deg}C. However, at 400{\deg}C, twist
angle relaxes significantly, accompanied by a decrease in spatial uniformity.
Uniaxial heterostrain initially increases and then decreases over time,
becoming increasingly non-uniform in direction. Structural irregularities such
as step edges, contamination bubbles, or contact with the underlying substrate
result in local inhomogeneity in the rate of relaxation.
