I finished a custom kit a few months ago, so I can share how it went for me. I also only worked on it in short evening sessions, usually around 40 minutes at a time, sometimes even less if I was tired. It didn’t feel overwhelming at all, because the canvas is still divided into small numbered sections, so you can just pick a color and continue from wherever you left off. The only thing that made it take longer was that I kept taking breaks because I didn’t want to rush it. Overall it took me about ten days of casual work, and I never felt like I needed big blocks of free time. I actually think shorter sessions made it more relaxing, not less.