Command line command to auto-kill a command after a certain amount of time

GNU Coreutils includes the timeout command, installed by default on many systems.

https://www.gnu.org/software/coreutils/manual/html_node/timeout-invocation.html

To watch free -m for one minute, then kill it by sending a TERM signal:

timeout 1m watch free -m

Maybe I'm not understanding the question, but this sounds doable directly, at least in bash:

( /path/to/slow command with options ) & sleep 5 ; kill $!

This runs the first command, inside the parenthesis, for five seconds, and then kills it. The entire operation runs synchronously, i.e. you won't be able to use your shell while it is busy waiting for the slow command. If that is not what you wanted, it should be possible to add another &.

The $! variable is a Bash builtin that contains the process ID of the most recently started subshell. It is important to not have the & inside the parenthesis, doing it that way loses the process ID.