How much time should we take to complete per topic with DPP?
There is not a particular or exact time that you should take to complete a topic. If you feel you are good at a topic and can easily solve most of the sums asked in that topic you can spend less time on that ,and focus more time on solving those topics where you feel you need to work hard on or that requires more practice.
Giving a little more time to topics which you feel are complex and where you need more practice on would be beneficial.