Hi Juan

English Language Pedant here

>Think of a data entry form where, among other settings, users need to specify the amount of seconds certain picture should be shown.

FYI, that should be "... number of seconds ..."

In English, things you can or possibly could count, is "number of " or "many" (as in "how many men in a baseball team", "the number of people who have asked me this")

Things you can't count is "amount of" or "much" (as in "how much sand is on the beach", "I'll have the same amount of ice cream")

Similarly: "fewer" and "less" ("There are fewer people here tonight", "There's less water in the bowl")

Thought the above may help you with other user system messages you may have to put up :-)

Terry

PS Oh, and "For" WAS the right answer.