It really depends what you need it for, 25% extra would be another 15mins in an hour exam. If you were nearly finished in the hour but just needed to do abit more and read it through that would help, but if you were no where near done at the hour then another 15mins might not help that much.
I didn't have 'extra time', but I was allowed breaks if I needed them. I had a bone tumour in my spine during my a-levels and I found it very hard to sit still for the exam, sitting for any length of time was agony, I was also on so many pain killers I was really drugged up. I was allowed a rest break (to move around etc) 15mins every hour. Although this helped abit I found it broke my train of thought, so it took me longer to get back into writing when I sat down again.
Anyway, thats off the point, what I mean to explain by that is that 15 mins/hour seems to be the normal that colleges give for extra time/special circumstances etc. If you find that you need longer (I did) then you might be able to apply for more, and some colleges may be helpful with it.
Don't know if it would be relevent or helpful without knowing what you need the extra time for, feel free to ignore this if it isn't relevent! If you are really struggling to finish exams in the time, maybe you could work on improving your exam technique? Try doing practice exam papers within the time limit. Plan your answer before you start writing (if its long answers, english etc,) if you don't finish answering it the examiner marking it can take into account notes etc (if you don't scribble them out.)
Hope that is of some help, and that I havn't just waffled!!