'When do you eat this' almost implies that you're asking them when it would be right to eat it, as if there's an allocated time, such as a festival or a special day. 'When would you eat this' has the exact same meaning but in addition to this seems more natural and can be used to ask someone when they would eat something: i.e a certain time. e.g When would you eat this? ( Roast turkey) I'd usually eat it at Christmas. Hope I could help :) I'd just use 'when would' because it seems more natural and less abrupt.

The difference is very small. Both sentences mean the same thing in most situations. Both sentences are grammatically correct.