What time of year would someone declare their earnings on personal savings? And how much?

My mother does not work currently and receives around 276 a fortnight for Employment Support Allowance. She receives around 235 a month pension from the NHS and 100 a month for bereavement support allowance.

She has money in her savings accounts that’s generated well over £1000 since April 2024 - is her allowance greater than 1000 because she isn’t working? She has never done any tax returns or anything before and would be caught off guard by fines if I don’t get involved. January is here and I’ve got that worrying feeling building that it’s not all in hand.

If she owes money to the state, how and when do we declare it?

Thanks in advance !