I have a fairly long bike commute (16 miles) and usually takes me about an hour...plenty of time to get pretty cold by the end. If I don't shower within about 5-6 min of arriving to work, I know I will be chilled to the bone ALL DAY. I have the warmest winter riding boots ever, and super warm socks, but my toes are still always super cold after about 45 min of riding in 5-10 degrees. SO my gauge for shower length is I stay in until my feet are warm! If my feet are still cold, even after a warm shower, I will seriously be cold all day. Also I NEVER wet my hair in the shower at work, that is another way to be really cold all day... Next the key is what you are wearing. I get a lot of crap for wearing "outerwear" inside at work. I just don't care...I always have a down jacket or vest, wool socks, a wool headband, and at least 2-3 warm shirts to layer if I need them. We aren't allowed to wear hats inside, so the wool headband is my way around that while keeping my head warm. If you are really still cold with all of that clothing, consider some of those chemical hand/feet warmers, they can really help!
Also, hot beverages/food are also key. I try to eat foods for breakfast/lunch that are warm, and filling. I save things like salads/fruit for home and dinner. I have coffee when I get to work, and also tea throughout the day if I am really struggling to warm up.
Space heaters can also be good! At my old job I used to keep one under my desk to keep my ever freezing feet warm...