It's still not clear to me whether you are considering Stimulus (with two levels, A and B) as an explanatory variable. In other words, are you interested in comparing the two stimuli, or do you just want to analyze each stimulus type separately?
Regarding modeling the variability of visit intervals, what kind of hypothesis are you envisioning? Suppose that, among the three consecutive visits for a subject, the time intervals (between visit1 and visit2, and between visit2 and visit3) are 4 and 7 days. First of all, what kind of impact of an interval on an upcoming visit? Linear? Secondly, how about visit1? Are you going to assign 0 days or something else (e.g., infinity) for it?
Gang