My two professors and I from undergrad, who I considered mentors from working in their labs and taking their classes, have different "keeping in touch". My one prof, who I thought I would be more in contact with, has actually gone the opposite. It's more or less the "how's everything going? let's get the old lab together for dinner" every once in a while. My other prof, who I thought she and I would never keep in contact with after undergrad, just 'cause I didn't think she really liked me haha has gone the opposite way. I'm still working with her on her studies, in fact, I'm helping her conduct a follow-up to the cohort from last summer during this summer. We're talking about collaborating and ever since I've graduated, I started babysitting for her and whatnot. I guess it just depends on the relationships you've cultivated from the beginning. For both of them, it's definitely more on a colleague level instead of professor/student now.