Easy or Hard Problems for Tactics Training?

Lately I've been working on my chess tactics. Every day I solve a couple of what I consider difficult chess puzzles (15-25 minutes to solve each). I was wondering though if it's better to solve a large number of simple problems (ex. 15-25 seconds to solve) instead or if someone has some general guideline for the ratio of difficult to easy problems.

Hard puzzles helps you improve concentration and calculation ability. Easier puzzles lets you see more patterns in a shorter amount of time. So I'd say they both have value and you should not exclude one for the other.

Difficult to guess a time ratio.... maybe 50/50?

There are puzzles even harder than the hard problems on Tactic Trainer.

Tactics from a good correspondence game are often more difficult than the problems on TT.

I agree with Louis-Holtzhausen that you should study easier and difficult problems on TT.