In order to comply with the rules, you have to spend at least 15 minutes working on the Day 20 Contest Round to get Merit Badge credit for Day 20. The guidelines do not include a maximum number of minutes, and I'm sure most Contest Round competitors spend much more than 15 minutes on their submissions. Therefore, the extra time you've spent here is not out of compliance with the guidelines - again, so long as you spend at least 15 minutes on the setting description for Day 20, which is required for October Merit Badge eligibility.

Good luck in the Contest Round.

Regards,
Michelle