How do I get just real time value from 'time' command?

time can take an optional --format or -f parameter, but you have to use the full path to the time command, /usr/bin/time

/usr/bin/time -f "%e" sleep 3

3.00

Without the path, it'll use the time command of the shell which just treats all arguments as the command, so you'll get an -f: command not found.

If you write \time you enforce not to use the bash built in time. So with \time -f '%e' command you are done.

If you're using the Bash builtin time, set the TIMEFORMAT variable to %R:

$ TIMEFORMAT=%R
$ time sleep 1
1.022