How often to meditate and how far apart

I am able to sit in floor in half lotus pose and meditate 30 minutes at s time. Originally I did once in morning and once at night but I have been looking to also do once before lunch and once after lunch (at least when I'm home from work) making it 4 times, total two hours. At this point I am trying to determine how to do the two mid-day meditations on the days I go to work. I am thinking perhaps once in morning, once after breakfast before I get train, once when I come home from work and once at night. Do meditations need to be spread out? And what happens if I am too sleepy to do meditation that I must skip else I will fall asleep during meditation? Thanks !