While many authors aspire to reach a point in their career where they can earn a living from their writing, most writers have to hold down a day job. Without delving too much into the indie versus legacy publishing routes (there are plenty of other blogs that tow the line for either path), there certainly is an opportunity for writers to earn something from their literary labor. Labor is the operative word here, as trying to write a novel while working is not easy.
Chances are that you've developed some time management skills during the course of your career or profession. Half the battle is showing up on time. By tapping into this skill set you should be able to take an objective look at your day to determine where you can carve out time to write. This may mean determining whether you have the time to write either before and/or after work. Likewise, you might have the ability to find time during your lunch hour, depending on your work situation. The demands of every job are different and certainly there are those where the idea of the traditional "lunch hour" is elusive at best. With that in mind, if you are going to try and write during your lunch hour, use your personal laptop or bring your own pen and paper.
Ultimately, it comes down to when is the most productive time for you to write. I'm a night owl, and need to write in a relatively quiet space. So I tend to write in the evenings after my kids are in bed or winding down for the night. You might be more of a morning person, so setting aside some time in the morning before work might work better for you. Just as with exercise, it's good to start out small: set aside 30 minutes a day, every day, to work on your current project. As busy as you may be, there's no reason why you can't find 30 minutes each day to write.
There's a temptation to devote all of your writing time to the weekends because you're just too busy during the week. The problem here is that if you are also leaving the weekend to take care of your personal errands, leisure activities and also work on the weekends, your writing time is still competing for time in your packed schedule. Likewise, writing is work; when you realize that, you stop seeing it in terms of a hobby or leisure activity. This makes it easier to carve out time, just like you would for work.
The point is to develop a schedule that incorporates your writing into your life. Like getting ready for work or your nightly ritual for going to bed, the goal is to make carving out writing time a routine habit.