There’s no hard and fast rules, whatever their normal way of working is in their file note. For us most SRB needs are ASD/ADHD related so a time out 1-2 times a lesson/exam is their norm, so we set 5 mins per half hour of exam as a guide and remind them they often only need 5 mins per hour and that they don’t have to have a break if they don’t need it. And also remind them that they have to ask for them too. Some students have expected to be sent for them in the past so it helps to be clear. There are no hard and fast rules but pre defined limits are useful for most to stop them being in the exam for hours on end, medical needs are obviously different.
When you say can’t get 25%, do you mean not nwow or don’t have the scores? You can have both if they have a diagnosis (or you have evidence of referral acceptance) and it’s their nwow.