In MA training, if you have to force yourself, you will quite someday. You train MA because you enjoy of doing it. This way, when you are 80, you will still enjoy your MA training. You should not set any time limit for yourself.
When I was young, I committed myself to spend 10 years to develop my "head lock". Everyday I forced myself to hang on the pole for at least 1 hour everyday (break it apart in many sessions). Oneday I stopped watching the time. I just did it until I no longer enjoined of doing it. Today, I have spent more than 35 years on my "head lock" development and I'm still working on it everyday.
The most important thing is "will you still enjoy of doing it again tomorrow, day after tomorrow, the next day, and ...?"
I have to force myself. By forcing myself that's how I improve and not just with martial arts but anything I choose to pursue. That's what taking on a challenge is all about. Sometimes we choose to make things hard because we want things to be hard, we want a good challenge. Even with stuff such as video games, a fun pastime of mine, I would sometimes play it on "hard" because I want the challenge. If I don't challenge myself I get bored with what Im doing. As a coach once said that in practice you've got to push yourself to your limit, and you've got to go a little bit beyond that limit, and this wasn't even a martial arts coach who said that it was a swimming coach, but the same concept can be applied to anything. I've got to push myself and take on challenges, otherwise I would be a "bump on a log," and that's not something I would be satisfied with.