I think it is more than enough time I have seen artheroscllerosis develop in diabetic mice in 24 weeks.this artile may help

Effect of Streptozotocin-Induced Hyperglycemia on Lipid Profiles, Formation of Advanced Glycation Endproducts in Lesions, and Extent of Atherosclerosis in LDL Receptor-Deficient Mice

Peter Reaven, Shiva Merat, Florencia Casanada, Mary Sutphin, Wulf Palinski

Wild type won't get much in the way of athero. Better to use ApoE ko  or LDL Kong 

The wild-type C57BL/6 mice supplemented with saturated fat, cholesterol, and cholate develop only a moderate lesion. An alternative to the ApoE and the LDLr KO models is the new developed AAV8-PCSK9. 

http://circres.ahajournals.org/content/114/11/1672

Wild type will not develop atherosclerotic, no matter the type of intervention you will use. You must use modified strains of mice, as the ones pointed out by the previous commentaries

Thanks all