For technical support of the items themselves, you will be responsible for responding to any bugs or issues with installation for 6 months after the buyer purchases it.
No custom work
You're not expected or required to do custom work, installation, or anything that goes outside of maintaining the theme itself and answering basic questions. Should you decide to do any custom work for a buyer all facets of that (coordination, payment, contracts, etc.) should happen off of the MUI Store platform.
When you're setting up your profile you'll provide a link to either your support site (such as Zendesk, Freshdesk, Front, etc.) or a mailto:email_address. We don't have specific guidelines on how quickly you should get back to customers but a rule of thumb is the quicker you came back to the customers, the better. Before 24 hours starts to be great a great experience on business days. No answers in 72 hours lead to poor user experiences.
We don't accept items without support because user feedback is critical to differentiate in the long run. You can decide to provide "light" support, however, customers will notice and leave bad reviews for the "Support" dimension.