When...? and How long...? For and Since

When = quando

How long = por quanto tempo

For = por

Since = desde

Ex: When did it start raining?

      It stated raining  an hour ago  /  at 1 o’clock.

Ex: I’ve been waiting for two hours.

      I’ve been waiting since 8 o’clock

For – not usually in negative sentences.

We do not use for + all… (all day / all my life)

Ex: How long is it since you last saw Joe?

       Quanto tempo faz desde que você viu Joe pela última vez?

  

Reference: English Grammar in use

Raymond Murphy