You need to look at the clock very regularly.
Typically, if I’ve spent 2 minutes on a question and I know there’s still half of the paper left, I already panic and think I’ve spent too long there. Especially when sometimes I see that 5 parts down there’s something I definitely know how to do. The only time I’d spend more than 2 minutes on a question in the early and middle phases is if I know that the answer is just long to write out and I’ve already done heaps.
Grab the easy marks first. Once you’re done, as you go towards the end, focus on those that you skipped. And balance it out, e.g. another 3 minutes per hard question thinking about how to approach it, or anything you can write down that would earn pity marks.
And of course, if later you realise how to complete that part in its entirety, go for it.