And they say, "When is this promise, if you should be truthful?" (36:48)

Almuntakhab Fi Tafsir Alquran Alkarim

And incidentally they add: When does that alleged promise – Hereafter – fall due, if indeed you are declaring the truth

Tafseer Tafheem-ul-Quran by Syed Abu-al-A'la Maududi

(36:48) They *44 say: 'When will this threat (of Resurrection) come to pass? Tell us if indeed you are truthful.' *45

And they say, "When is this promise, if you should be truthful?" meaning

*44) After Tauhid the other question about which a dispute was raging between the Holy Prophet and the disbelievers was the question of the Hereafter. Rational arguments about this have been given in the end of the discourse. Here, before giving the arguments, the Hereafter has been depicted with all its horrors so that the people should know that what they are refusing to believe in cannot be averted by their denial, but they have to meet and experience it one day inevitably.
*45) The question did not mean that they wanted to know the exact date of the coming of the Hereafter, and if, for instance, they were told that it would take place on such and such a date in such and such a month and year, their doubts would have been removed and they would have believed in it. Such questions, in fact, were put as a challenge only for the sake of argument. What they meant to say was that there would be no Resurrection whatever, as if to say, "You are threatening us with Resurrection without rhyme or reason. " That is why in reply it has not been said that Resurrection will take place on such and such a day, but that it shall come and shall be accompanied by such and such honors."
 

And they say, "When is this promise, if you should be truthful?" meaning in Urdu

یہ لوگ کہتے ہیں کہ "یہ قیامت کی دھمکی آخر کب پوری ہو گی؟ بتاؤ اگر تم سچے ہو"