Let   $K$  be a function on a unimodular locally compact group
 $K$  be a function on a unimodular locally compact group   $G$ , and denote by
 $G$ , and denote by   ${{K}_{n}}\,=\,K\,*\,K\,*\cdots *\,K$  the
 ${{K}_{n}}\,=\,K\,*\,K\,*\cdots *\,K$  the   $n$ -th convolution power of
 $n$ -th convolution power of   $K$ . Assuming that
 $K$ . Assuming that   $K$  satisfies certain operator estimates in
 $K$  satisfies certain operator estimates in   ${{L}^{2}}\left( G \right)$ , we give estimates of the norms
 ${{L}^{2}}\left( G \right)$ , we give estimates of the norms   ${{\left\| {{K}_{n}} \right\|}_{2}}$  and
 ${{\left\| {{K}_{n}} \right\|}_{2}}$  and   ${{\left\| {{K}_{n}} \right\|}_{\infty }}$  for large
 ${{\left\| {{K}_{n}} \right\|}_{\infty }}$  for large   $n$ . In contrast to previous methods for estimating
 $n$ . In contrast to previous methods for estimating   ${{\left\| {{K}_{n}} \right\|}_{\infty }}$ , we do not need to assume that the function
 ${{\left\| {{K}_{n}} \right\|}_{\infty }}$ , we do not need to assume that the function   $K$  is a probability density or nonnegative. Our results also adapt for continuous time semigroups on
 $K$  is a probability density or nonnegative. Our results also adapt for continuous time semigroups on   $G$ . Various applications are given, for example, to estimates of the behaviour of heat kernels on Lie groups.
 $G$ . Various applications are given, for example, to estimates of the behaviour of heat kernels on Lie groups.