how often should you mix up your workout?

I was wondering today while at the gym, how often to change my work out up, i mean for example switch from flat barbell bench to dumbell bench. I i know you are supposed to switch things up , but what happenes if u dont ? will you get bigger slower than by switching it up?
}I was wondering if this is enough switch, say i do 2 weeks , or i should say 2 days for each muscle then for the 3rd day switch to another , then for the 4th go back
for example
1st day for chest - flat barbell bench
2nd day for chest - flat barbell bench
3rd day for chest - flat dumbell bench
day 4 and 5 same as 1 and 2 , and so on

let me know if this is good enough , i want to get the best out of everything , and i dont want to use the same exersise every week, so let me know if im on the right track, thanks

I change barbell,dumbell,machine on a daily basis, but i change the order of my routine every month.

everytime i hit the gym i never do the same excerises as prior or i always change the order in each workout. so every trip in the gym is something different everytime

I pretty much change my stuff up every time I step into the gym, sometimes I even change that and do the exact same workout twice. I like to try everything, KEEP YOUR MUSCLES GUESSING!!!

Pain

I'll agree with pain and Yung, however there are some things that i don't change. For example, i will always use DB for bench press because i can flex my chest much harder then if i used a bar, which i beleive is more important for me. So, what you change from workout to workout is up to you, but i would be changing something every workout. Rather it be # of reps, rest intervals, sets, exercises, etc.

Yup, there are a few staples of my workouts that only get changed once in awhile, they are the tried and true exercises that always give me results. They still get changed sometimes though.

Pain

I kinda have an "A" week and a "B" week that rotate constantly. Of course I'll probably switch that up in a month. But it's been working for the past 2 months. Just as soon as my gains quit coming or I don't feel the smae type of pumps or if I just get plain old bored I change it up.