Use Case:
Send Email to Account Owner every hour until the Account Type is Prospecting.

Custom Field:

Process Builder:

Object Criteria:

Send Email True Criteria:
When Account type is Prospect and Send Email is True, then
Time Trigger: 1 hour after last modified date
1. Send Email
2. Update Send Email check box to false.

Send Email False Criteria:
When Account type is Prospect and Send Email is False, then
Time Trigger: 1 hour after last modified date
1. Send Email
2. Update Send Email check box to true.