With WTD, you can see lesion formation already at 6 weeks, to be on safe side I would keep them on diet for 8 weeks.
Nakashima,Y, Plump,AS, Raines,EW, Breslow,JL, Ross,R: ApoE-deficient mice develop lesions of all phases of atherosclerosis throughout the arterial tree. Arteroscler Thromb 14:133-140, 1994