I find it hard too :/

Rule of thumbs:

1. Show errors immediately if issues are severe
2. Late validation is almost always better
3. Validate empty fields only on submit
4. Reward early, punish late: if a user edits an erroneous field, we should be validating immediately, removing the error message and confirming that the mistake has been fixed as soon as possible (reward early). However, if the input was valid already and it is being edited, it’s probably better to wait until the user moves to the next input field and flag the errors then (punish late).
5. Validate after a copy-paste
6. Allow users to override inline validation
7. Just in time validation
8. For short forms, consider validation on submit only. For complex forms, use the task list pattern