Hi No, it is best to ask, How long is it till my/our dinner is ready please? or Is it a long time untill dinnertime? When will dinner/tea/supper/breakfast be ready please? this last one is probably the most common one. You could also substitute dinner /lunch etc for 'meal' or 'snack' if it is a snack. Best wishes Gillian

No, sorry... your sentence sounds as if you want to know how long dinner time will last (and it still sounds unnatural). Your magic word is "until": "How long (is it) until dinner?"