  Let $G=(V,E)$ be a locally finite connected graph. We develop the first
eigenvalue method on $G$ introduced in 1963 by Kaplan \cite{Kaplan} on
Euclidean space, the discrete Phragm\'{e}n-Lindel\"{o}f principle of parabolic
equations and upper and lower solutions method on $G$. Using these methods, we
establish the estimates and asymptotic behaviour of the life span of solutions
to a semilinear heat equation with initial data $\lambda\psi(x)$ for different
scales of $\lambda$ on $G$ under some different conditions. Our results are
different from the continuous case, which is related to the structure of the
graph $G$.
