Average solving time // Puzzle Difficulty approaches (?)

Just wondering what is the average time it takes someone to solve a puzzle that is on their hardest level more or less (without being over-rated for them / extremely hard)

I heard contradicting approaches on what is the most effective and improving way to solve puzzles; some say 

• solve as many as you can with each puzzle taking you about 10 seconds to 1 minute to solve

while others say 

• do 4 puzzles in an hour (15 minutes each)

I assume a balance of easy and hard ones is best, but curious about other people's routine. I guess puzzle rush is an effective tool for speed chess, but as for true chess skill improvement (rapid/classical) I'm curious as to what people think is the best way. 

P.S; it just took me 25 minutes to solve this: https://www.chess.com/puzzles/problem/1263013

it felt really satisfying to solve it and find all the resources for the opponent..  it normally takes me 15 minutes to solve a 1900-2300 puzzle... 

The puzzle you talked about, it took me less than 10 seconds ;-;