I am an attorney and have practiced both with law firms and corporations. The hours required will depend on many things. First, the type of law that you practice. If you work in litigation, then there are heavy hours right before trials or large hearings. There are also mediations that can last all day and into the evening. If you work in real estate, the long hours come at the time you are closing big deals (and this often happens at the end of the year for tax reasons). Different practice areas may require heavier loads at certain times of the year.
My experience has been that practice with law firms have longer hours, because the attorneys have to bill so many hours each month and they may need a paralegal's assistance. Of course, some corporate cultures tend to push everyone to overtime, because of being short-staffed.
As with any job, you can find ways to make the hours work. I know some paralegals who work from home doing transcriptions and some research. They are able to plan their days and only bill the hours they work.
I wish you the best in whatever you decide. Even if you decide not to practice strictly as a paralegal, it gives you skills that you can use in any position.