How long does it take for antidepressants to fully kick in? and does it take longer the second time using them after a few months?

AD's.: Usually 4-6 weeks, probably less when used second time.

Depends: Upon the specific antidepressant and on metabolism. Usually 4-6 wks to see full benefit, but should see some benefit before that. Placebo effect plays a huge roll so may feel better just because you are doing something. Actual therapeutic benefit takes longer. Combining therapy with antidepressant is even better