In Quant, what I tell my students is that with very few exceptions, you should never spend less than 0:45 or more than 2:45 on a problem.
0:45 is the low end, because that's how long it takes to read a problem, think about it, and determine whether it's worth your time. That's probably more time than you're currently spending on the 'read and think' step - possibly a lot more time. Try this: at the beginning of each Quant problem,
put your pen down completely and make yourself do
nothing but reading and thinking until 45 seconds have passed. At first, it'll seem like a useless exercise, but with practice, you'll start to realize what sorts of things you can notice and think about during this time. Also, when you start actually doing the math, you'll be more likely to have a good plan, which means you won't be tempted to go back and try another approach (something you rarely have time for on Quant.)
If you get to the 0:45 mark and you don't have a reasonable idea of how to do the problem (note that I didn't say 'don't understand the problem' - even if you understand a problem, it still might not be worth doing!), spend another 30 seconds coming up with a smart guess, then guess and move on.
If you
do commit to the problem, reevaluate as you approach the 2 minute mark. Is your plan working? If not,
don't let yourself fall for the 'just another few seconds!' trap. A few seconds is good when it means you'll get the problem right. So, if your plan is working, and you just need a little extra time to wrap up the math itself, its okay to go up to the 2:45 mark (or even a little further if you're ahead on time.) But if you're having a hard time because you don't know whether you're doing the right thing, or because your plan is failing and you need a new one, don't let yourself say that you only need another few seconds. That isn't enough time to come up with a better plan
and implement it, so you'll be better off just moving on.