I think it was nearly immediately after the wedding. I remember thinking she was probably been totally doped up while being courted -- I vaguely remember reading about Rochester talking about how out of control Bertha was during their honeymoon period. (I wikied it and it's towards the chapters 20-26 that shit blows up in his face and he has to confess to Jane, in case you have a copy handy.)

I'm assuming it got progressively worse. I googled (magical google!) her timeline: http://www.shmoop.com/jane-eyre/bertha-mason-timeline.html (http://www.shmoop.com/jane-eyre/bertha-mason-timeline.html). It looks like they were married for 4 years where he tried to get her treatment, but ended up locking her crazy ass in Thornfield.