How many MGs a week? Well that depends on how much you want to take.
I'd advice that you take some sort of Test with it... DECA will shut your test levels down, and you wont be able to get an errection.
If thats not important to you, then go ahead and run deca alone.
If I were just going to run it alone (which i wouldnt) I'd ran at least 500mg a week.
Make sure you have proper post cycle therapy (pct) lined up. You're gonna need a lot of Clomid to get your boys back up to par after this cycle.