I'm afraid there is no standard. I've gone through easily over 100 past papers published by AQA from GCSE up to my second year of A-Levels right now and if there's one thing I've learnt you're at the mercy of the mark scheme. This is from doing Maths and Physics (Applied Physics as optional module, so a lot of calculations in that one alone).
I've learned some trends though, most of the time you round to the same number of SF as the values your given in the question.
However for questions carrying on involving your answer to a previous question like 1. a) i) to ii) it varies a lot. For questions where i) is a 'Show that this is x=1.2' question you should round to 2 more significant figures than the answer they give (x=1.189), so that in the next question ii) when you have to use the same value of 'x' you use the answer you calculated and not x=1.2.
There are times where you have to make a judgement call. Like in Simpson's Rule in maths to find the area under a graph I always round 3 SF and have another answer next to it in brackets showing 5 SF. Or when working in units of MeV I always round to a whole number, showing another answer next to it to more SF.