Use only "tempo da perdere".
The only thing "a perdere" I know here are bottles. In some stores you can bring back wine's or milk's bottles receiving few cents for them. When you don't have to give them back, and you can only throw them in the dustbin, then you say that the bottles are "vuoto a perdere".
as a general rule, without reading any context, you can consider "di" an "of" and a "da" a "from". This doesn't mean they are used always in the same way.
Sometimes this general rule has relevant exceptions. The more famous exception coming to my mind right now is "Where are you from ?" This becomes "Di dove sei ?". The reason for this is that the italian question has more the form of "of what place you are ?".
Instead if you ask "Where do you come from ?", things go back to normal, and you translate with da (from): "Da dove vieni ?"
All this will appear to be rational if you substitute "dove" con "che posto" (what place). Hence it makes sense to say:
- Di dove sei ---> di che posto sei ? ----> of what place you are
- Da dove vieni ---> da che posto vieni ? -----> from what place you come
In other sentences, as a simple "Where are you ?" translated with "Dove sei ?" dove can be thought of as meaning "in what place".