If you are using Sustanon alone I would inject twice a week.
Susta is a combination of 4 testosterones - short and long acting. Because of the proprionate, susta is effective after one day, and based on the mixed in decanoates, remains active for 3-4 weeks.
If you combined with deca or D-bol you could perhaps leave it with once a week. It also depends on whether this is one of your fisrt cycles, or whether you are further down the track.
Good luck and enjoy that pump!