Here is my small suggestion.
Every aspirant is at a different stage of preparation and he/she should utilise this time accordingly. There needs to be a macro analysis first, before we delve into the micro.
Here are a few possible scenarios:
1) If one is fairly comfortable with the prelims preparation, it is imperative that one must focus more on Mains at this point of time especially the portions that were neglected before one started into full time prelims preparation. The priority here should be :
a) Optional
b) Ethics
2) If one feels that he/she is on the margin in the prelims preparation, then it is important to first focus on the prelims rather than running behind the Mains/optional. This time can be utilised to strengthen those subjects which have traditionally given good cost:benefit returns:
a) Environment with backward forward linkages with CA
b) Economy with special focus on CA
c) Polity
d) Modern History
If any of the above portions still need time, then it is worth the investment.
Depending upon one's confidence and analysis most aspirants lie somewhere in between the two extremes of the spectrum. One has to choose what is best for him/her.
Additional points:
1) Prelims is qualifying, but every mark scored in Mains counts.
2) Given the uncertainty, I personally feel that UPSC will keep delaying it with only limited days at once.
3) It will not be useful to spend this time to focus upon new topics or books for prelims which one had left thinking that it might give less cost:benefit. For example refrain from picking up an ancient history book for prelims now just because you feel you had not done that subject earlier.
3) If one has done every aspect of prelims 4/5 times then little break is also okay so that it doesn't get too monotonous.
4) Last month revision will be very crucial and that energy will be very important to cross
All the best
