  Kinetically constrained spin models are schematic coarse-grained models for
the glass transition which represent an efficient theoretical tool to study
detailed spatio-temporal aspects of dynamic heterogeneity in supercooled
liquids. Here, we study how spatially correlated dynamic domains evolve with
time and compare our results to various experimental and numerical
investigations. We find that strong and fragile models yield different results.
In particular, the lifetime of dynamic heterogeneity remains constant and
roughly equal to the alpha relaxation time in strong models, while it increases
more rapidly in fragile models when the glass transition is approached.
