Depends on the meds and how you respond. Going on & coming off MAOI's save 14 day washout periods, meaning you can't take anything new 'til that's out of your system. Otherwise, as people have already pointed out, you set up a cross-taper over time until you're off the old med and on a therapeutic dose of the new one.

I kinda recently went from 225mg Venlafaxine XR to 80mg Duloxetine in 2-3 weeks (maybe 4) if I remember correctly.