Having 5x 15x and 2x seems a good balance.

35 days

hmm depends, presonaly like x 1.5 is best

Depends on if you have different levels of rate. For an example

• Easy - 10 to 50 times faster than official 
• Med - 5 to 25 times faster than official
• Hard - 2.5 to 12.5 times faster than official  
• Real - Official rate.

It all depends on what you want to have if you want to make it more grind or have a relax time or bit of both.