I have a little Moleskine (3.5 x 5.5) notebook that I only draw skulls in. I started in November of 2013 and I do one whenever the urge strikes me. It's not like a skull-a-day thing but sometimes I do get into a period where I will draw one every day for a while then I won't draw any for months. I even lost it for a while and was very sad. I think the longest gap between pages has been a year. This is the most recent skull, drawn on 05.28.2018. Most of them are posted on my Instagram but you have to scroll back a ways to get at the bulk of them.