From my experience after 6 months my true form will start to show through. I find myself withdrawing from others as a way to protect my image. After the 6 month period. I go into a kind of neutral form, not revealing my true self and not quite breaking character. I can maintain this form for quite some time.
I don't have any character role I go into. I don't become someone else as such. I just fake certain emotions when required and lie when I want to, for various reasons.
I may lie about certain aspects of myself as well, but not to the point of becoming someone else entirely.
It's a social mask that displays a side of me that isn't sincere and it lasts as long as I need it to in social interactions, unless someone pisses me off. Then the mask slips.
How about all the time?
At one point I had a triple life. I was a drug addict, I was lying about my sexual orientation and well I was faking certain emotions (as TK mentioned) so people wouldn't think that I'm a freak/cold-hearted.
That's so hard to manage because you have to remember specific lies you told to specific people.
Nowadays there's only one layer left and I'm always 'playing' someone easygoing and sociable even though I'm not really.
Sometimes my mask does slip especially when I get angry but in that case I would typically avoid people if I can.
I even want to a psychiatrist to get that anger under control because I don't care about being a sociopath really but I do care about having control.