Crontab is intented to be used with scripts, that execute a task and the exit. For example, if you'd like to collect some data every 10 minutes and send it to a remote server. Crontab will simply execute this script every 10 minutes.
If the last cron job has not finished running, crontab will run it again, creating a second instance of that script or crashing the first one (depending on the script).
Perhaps you could commendpt more about your use-case to see if we could find the best option?