Whenever you feel like it
It's a very different scenario when you study an instrument as an adult. It's great that there's no one monitoring you and making you practice.
On the other hand, there's no one monitoring you and making you practice. So what will happen?
You’ll play when you feel like playing.
The "all or nothing" thinking that some of us carry with us from childhood is not helpful here. If you miss a day of practice, it's no big deal. Just get back into it the next day, or the day after. You don't have to prove anything to anybody.
If you keep picking the instrument back up time after time, even if it's not every day, you will make progress and you will have fun. That's what it's all about.
If you don’t — if your enthusiasm eventually wanes and you never feel like playing — well, that’s fine, too. It doesn’t make you a failure. You’re simply choosing to do something else with your time.
You may feel a disconnect between what you wish for yourself in the long term and what you are willing to do in the short term. In other words, the discomfort of not already being good at the instrument is too painful to tolerate, so you dread it.
In that case, you’ve got three choices:
Make the experience less uncomfortable;
Habituate yourself to the discomfort;
Accept that you don’t want to do what it takes.
To make the experience of practicing less uncomfortable, you’ll need to quiet the voice in your head telling you that you should know this already, you never had any talent or discipline, you don’t know what you’re doing, this isn’t going anywhere, there are more important things you should be doing, et cetera. Things get much easier when that guy shuts up.
You may also wish to invest in the help of a teacher or guide who can show you ways to make practice less painful and give you better, faster results.
To habituate yourself to the discomfort, keep your practice sessions small and frequent. Make a game out of noticing the moment when you are about to feel frustrated, and then stop immediately. It could be twenty minutes — it could be fifteen seconds. Your tolerance could build over time. Play with it.
And maybe you don’t want to do what it takes. That’s fine! I turned out to be a lifelong musician, but I really didn’t enjoy playing the trumpet in fifth grade, so I quit. I still don’t enjoy playing the trumpet. It’s not a character flaw. It’s a legitimate personal preference. It’s allowed.
When I give myself the freedom to practice something new whenever I feel like it, I eliminate the shame, resentment, and frustration that leads to procrastination and avoidance. Things stay light and fun. Once I’ve built a bit of momentum, then I can make a commitment, but that is meant to help me follow through on something that I want, not something that I dread. If I end up with something on my calendar or task list that I feel awful about, it’s time to renegotiate.
Life is full of obligations. We all have to do stuff we don’t want to do. We don’t need to add more layers to it. So if you feel like playing your instrument, play it. Have fun. See what happens.