Atleast 50 puzzles per day.
how much time should we spend at tactics daily?

No time at all, if I have a date.
Now on the serious side, not all puzzles are equal. I solve some withing 4-5 seconds, some others within 4-5 hours, and some not at all.
Solve as many as you can before feeling tired. Oh, and don't even think having electronic aid when solving- else you are just wasting your time, as well as several CPU cycles.
Whatever puzzle you are unable to solve, do NOT pass it to the computer. Archive it, and review it after 2-3 months. Chances are that you will be successful at your second/third attempt.
This would be the first time I read Pfren being funny. Opa!
I do 4 or 5 per day....when I have more time I do more....I guess it depends on each person time availability???
hmmm