According to some investigating by TYT: in order to request an official pardon according to DOJ Pardon Guidelines, a convict must have spent five years in jail. Arpaio at the most will get sentence to 6 months.

This might be why Trump's keeps saying he will make the announcement at a later date because he can't pardon Arpaio.

There's also a lot of other requirements that need to be met. For full video on TYT discussing this topic:
https://www.youtube.com/watch?v=841V0EHIesQ