I have deeply analyzed the issue as in subject, and I think I've found the (almost) perfect configuration for the maximum battery time:

Phonelopnik: Windows Phone 8.1 Battery Saving Hints.

Of course I read a lot of forums and a saw a lot of YouTube vids too, but here are my suggestions of which some are quite obvious and some are not:

1) Keep active in background the only apps that are really very important to you. If you can survive without being constantly updated about your Twitter and Facebook, kill them from working in background and they'll be updated automatically as soon as you open them.

2) For your email downloads (I have 4 mailboxes) select "update according to my usage". After many, many tryings this seems to be the best configuration. The mails are downloaded as soon as they arrive on the server, but if you rarely use one or two mailboxes, after sometime they'll be actually inactive.

3) Set "Battery Saver" to "NO". Looks like a contradiction in terms, but this app uses A FUCKING LOT of power just to constantly monitor the battery level, in order to switch off the internet interactions once it reaches the 20%. It's much better to keep it OFF, and switch it on once you see that your battery is becoming quite low. Just imagine that you are at school or at work, where you don't use much your phone: if you disconnect it from the charger in the morning, do not play games or see vids and just make a coupla phone calls and check the news at lunch break, well I bet a beer that at 7:00PM your battery will still have a good 80% charge left, which is not bad at all.

With the settings as above and being in stand-by (only one call of a few minutes), after 1 day and 6 hours I still have 79% charge left. All those who cannot reach noon with a charge, will probably like my suggestions... mainly the one about the Battery Saver setting.