When should you hire a consultant? This is similar to the questions of:
First you need to be aware of the issues of employee versus independent contractor. I will assume you understand the implications of that issue with respect to liability, taxes and workers compensation insurance.
I guess I would address the question this way. When would I hire a consultant? I would hire a consultant if...
If the need or project was full time or permanent, I would hire an employee to handle the job. If it was something I knew how to do, but simply did not have the time for, I would try to delegate some of my other responsibilities to a business partner or employee.
Hiring an outside person has several benefits. You can keep focused on what you do well instead of trying to learn a new skill. The outside person might have skills and experience that you do not possess. You will not have to pay the outside person benefits or withhold taxes. AGAIN BEWARE OF THE RULES OF EMPLOYEE VERSUS INDEPENDENT CONTRACTOR.
You can view hiring a consultant in more ordinary terms. My car breaks down. I hire a mechanic to fix it. I do this because:
While you may automatically come to this conclusion in your personal affairs, sometimes you will try to go it own your own with your business affairs. But as a small business owner you do not have the inside staff to handle accounting, information technology, human resources, finances, marketing, etc. and manage the business at the same time. It is perfectly reasonable to admit that you can not do everything.
I have to confess that I often spend hours learning things simply because I want to do them myself. I am being smart and stupid simultaneously.