Trying a new food? How long?

My cats a few ideas to try. But I have been reading other posts. I read that some allow weeks to see if it makes a difference. Is that true for diarrhea? 

I hate to see them suffer for so long. 

He is currently on a chciken/pumpkin diet. Doing good.  So if add say a grain free cat food. And starts to get soft stool right away? Doesnt that mean its not a good choice for him? Or could things change in weeks time? I know it stopped with the chicken and adding the pumpkin helped him poo. 

Yes, if you change his food and he gets soft stools immediately, then I would say it's the food. 

Why, may I ask, do you want to change his food if he's doing fine on what he's currently eating? Is he having other issues, or do you just want to feed him something better (grain free).  You say he is currently on chicken/pumpkin.  Do you mean just plain chicken, not chicken CAT FOOD?  or not something 100% nutritionally complete?  If so, then I see why you need to move him on to something else.

Are you giving him probiotics?  If he is having soft stools, probiotics could help with that, along with diet. 

If you have another thread concerning his issues, it might be helpful to link to it so we know his story