You can ask Allah for anything. :)
In fact we are suppose to ask Allah for all our needs big or small provided they are jaiz.
You Alone we worship. And You Alone we ask for help. Al-Fatiha:4
The best time for duaa is during prostration in salah when the servant is closest to His Creator. Dua in/after Tajjahud prayers and during Ramadhan the time when a person breaks there fast (Iftari).