I can't really speak for depression as a whole, because I think it has many different facets and so many varying degrees of effects.
I can only speak for mine.
I had it really bad for about a year, but i would say that I was up and down for 3/4 years before that. I felt like my whole personality changed.
I was so jokey and take the piss out of everything that went wrong, (and if something was going to happen, I was always the one it happened too) Then slowly over time I just lost my sparkle. I didn't even know I had, until I happened to look back at emails that were 2/3 years old, and I didn't recognise that person. She sounded happy, full of life, funny, fun.
I couldn't concentrate at all, trying to read an article in a magazine was furitless, never mind a book. Even writing this out 1 year ago, would have shown how choatic my mind was. I would miss out whole words in a sentence, and when speaking would stop half way through because I had forgot what I was talking about.
I got up every day for my kids. I made sure I kept on top of the washing and the food, everything else was as and when I could. I've lost friends because I couldn't face anyone. I dreaded having to entertain even family members.
I think the worse point came when I thought that i'd been let in on a secret, and the secret was that life is awful. So awful that making yourself live it is just stupid and pointless. I didn't get why others thought otherwise. 
And know this won't be popular, but through my readings and research I came across things that resonated with my own personal experience. One is that depression is anger turned inwards. I was angry, very angry at my dp. The other was that to depress is a verb.
I do think that I lived in my own head all the time. I'm a procrastinator, I have circular arguments with myself, I ruminate ALL.THE.TIME. and the worst is that I take no action.
I was on tablets for 8-10 months. It probably should have been longer, but they made me really aggressive and fogged.
In the last year of being off them, I have really tried to change the way I think and view things. Basically I try not to be in my head. I've been out for a night out once in the last year, and only 3 times in the last 5. But I have made a lot of changes, gym, lost weight, kids at nursery 5 mornings etc.
I have a bit more fun back in me, but it has come at a cost. I am a lot more detached from emotion. I rarely cry. And i don't think my dp is capable of hurting me as my heart is steel as far as he's concerned. 