Yes, but "take long" has a slight connotation that the person we're waiting for has control of the amount of time he will be gone.
Do not use the word doubt if you expect a reply. In most cases, doubt means "to lack belief."
If you say, "I have questions about your education," it could mean that you want to know where I went to school, what subjects I studied, etc.
If you say, "I have doubts about your education," it means that you think I am uneducated to some degree.