Question: At what times is one allowed to offer nafal prayers and how often can one offer sajdah?  
    Answer: Except for the time period during which the sun rises and sets, one can offer nafal prayers and sajdah. Sajdah is perhaps the pinnacle of prayers and as such has great importance. It can even done when one is not praying. The Prophet (sws) is reported to have said:  

A person is nearest to Allah when he does sajdah. So say a lot of prayers [in this state]. (Mishkat, Kitabu’l Salat)