How long does it normally take you to get over someone?

So a guy I was seeing on and off for six months is now out of my life for good probably. He was my first for pretty much everything. It's been a month now and I still feel like someone is standing on my chest when I think about him. How long does it normally take you guys to get over someone? For some reason I feel like I should be over him already since it only took him 3 weeks to get over me and go on to the next guy.