i would go 8 weeks max. deca will linger in your system for weeks after.. 8 weeks is really 13 weeks. i was still gaining 5 weeks after my last deca injection. thats why deca was so popular back in the old days IMO, it keeps working well after last injection. make sure you overlap test..
deca is probably the only AAS that i would never run alone without test as a base. i have run primo, tren, and other AAS without test and was fine... would not chance it with deca though