If you have full flexibility, I’d suggest aiming to hit 40 hours on a weekly basis rather than 8 on a daily basis. That gives you the flexibility to work, say, 10 hours one day because you’re really on a roll, and to stop at 6 hours the next afternoon when you’re tired and distracted without feeling guilty.
Don’t go over 40 hours on a regular basis. That’s a recipe for burnout. A week of crunch time every 3 months is one thing. But regular 50-hour weeks aren’t sustainable for most people.
I’d also suggest setting goals in the morning for what you want to accomplish in a day. Put down your work when you reach the goal, unless you’ve got several more hours in the day. In that case, set another goal for the remaining hours. If it becomes clear that you won’t reach your goal within a reasonable number of hours for the day, put your work down whenever you hit a stopping point close to the time you’d planned to wrap up for the day.
More experience should help you make increasingly accurate estimates of what you can accomplish in a day, so you’ll hone this process as time passes. Plus it’s great practice at estimating, which is a difficult and valuable skill.
It’s unusual to hire junior engineers fully remote in part for this reason: more experience in a “typical” work environment give you more context for these sorts of judgement calls.