  The process of $H$-bootstrap percolation for a graph $H$ is a cellular
automaton, where, given a subset of the edges of $K_n$ as initial set, an edge
is added at time $t$ if it is the only missing edge in a copy of $H$ in the
graph obtained through this process at time $t-1$. We discuss an extremal
question about the time of $K_r$-bootstrap percolation, namely determining
maximal times for an $n$-vertex graph before the process stops. We determine
exact values for $r=4$ and find a lower bound for the asymptotics for $r \geq
5$ by giving an explicit construction.
