How long do I have to submit additional documents for a DTV application after an embassy request?

Hi, Please can I ask -- When an embassy requests additional documentation for an online application for DTV, do you know how quickly we have to submit it? (I mean after how many days or weeks do they scrap/annul the application if they are still waiting for the additional documents?) I don't have some of the stuff they want immediately to hand, but I reckon I can arrange it, with a bit of time... I applied via London embassy (online). Thank you!

Also... They have requested a UK tax return or accountant's letter (or similar). They say I can black out everything bar my name. I haven't yet submitted my tax return for year from April 2023 to March 2024 (i.e. the latest year)... (It's due January 31 latest). So any tax return I give them would look more than a year old. I am thinking of submitting an accountant's letter, therefore. Do you know what the letter should say? Maybe she (my accountant) knows how these things are done... but in case she needs instructions, I'm wondering what I should say....