Hiya,
I have no idea how long we got to check the trains for faults. We had a laminated piece of paper with prompts and fault codes on it, then a 'question paper' which showed two train carriages and then an answer booklet where we had to write the answers.
You basically have to read through the question paper and see what the faults are, you then prioritise the faults in the most important order, check it against the laminated piece of paper which will give you the fault code(s) which you then write into your answer booklet. I think there were 20 in total, I got to 13 but I know I messed at least one of them up. I still passed with enhance score which was really surprising to me.
I never practised the DFFT and I didn't take it two years ago when I first did the assessment 1. Just stay calm, read everything, listen carefully and ask questions if you need to.