True it can take some time to get fully licensed and begin working independently. Working for someone else always induces some stress into your life because of their involvement and expectations. Once fully licensed and working independently the stress level is whatever you allow it to be. If you are organized, accept no more work than you can submit within the given time limits, select clients who are not PITA's, learn to schedule out of the office (family, recreation) time, etc. Overall it works out to probably to a range of 4 - 6. You can control the workflow, you can work a much or as little as you want, you can work early in the morning or late at night or all the time, you can work pretty much where you want (office, home, beach, cabin, etc.), you can work with or for who you want, etc.
Obviously there are days and times that things don't work out as expected and for a short period of time things can be very stressful. Probably the most stressful part of the work is being independently employed and learning to budget your money and adjusting to an inconsistent cash flow. This can cause you to accept assignments you really don't want to do or are not comfortable doing and to accept more assignments than you can complete within the required time frame.