Hazrat Moulana Ashraf Ali Thanwi (rahmatullahi ‘alaih) once mentioned:
If a person is addressed by any of his seniors (e.g. his parents, ustaaz, shaikh, etc.), then it is extremely disrespectful for him to ignore the senior and intentionally not reply to him. Similarly, it is disrespectful for him to answer them with a delay as delaying in replying will cause them some type of inconvenience. (Malfoozaat Hakeemul Ummat 23/82)
Source: Ihyaauddeen.co.za