To keep watch running indefinitely until it’s stopped manually, you can use the -n flag followed by a large number to make watch repeat the command at a sufficiently long interval. For example:
watch -n 9999 command
This will repeat the command every 9999 seconds, effectively making watch continue to run until stopped manually.
Note that there is no way to set watch to run indefinitely by default, as it is designed to run a specific number of times and then stop. However, using a large number with the -n flag as shown above can effectively achieve the same result.