What  often  occurs  lategame/endgame  is  that  you  have  strong  enough  champions  that  can  solo  content  making  you  able  to  bring  food  as well.  This  alllows  you  to  always  have  food  avaible.

You  should  focus  on  solo  clear  12-3  brutal  and  just  spam  with  your  campaign  farmer for  fastest  experience.

Honestly,  to  make  five  5  star  chickens  takes  a  good  while  and  a  big  chunk  of  energy.  With  regards  to  how  long  it  takes....  if  you  have  the  energy  then  not  long  but  it  does  take  a  lot  of  work.  

I have saved some brews around 400-500 each affinity, chickens from events and doom tower, and from other content that I probably I forgot to mention. 

So I 6 star 3 champions in minutes and max them to 60 without consuming energy

There are daily rewards in this game for 270 days. Things are supposed to take a long time.

I see 😀

It seems I will spend a lot time here... 🙉. However, 2 60 in month seems to be reasonable.

it all depends how fast and how much you play. The first level 60 should take arround 1 week if you know what you're doing. Then after that is much faster since you already have one solid campaign farmer