From my experience and subsequent understanding, setting a time limit doesn't really make sense and would, in most cases, inhibit the process.
The way I understand IM is that I already have whatever I want. The time it takes for the intention to manifest is merely the time "I" take to eliminate from my mind any feelings that that's not true and that I "need" to "manifest" it. I don't need to do anything, not even "try" to manifest. All I have to do is form a picture knowing that it already exists for me.
I came to this understanding after certain intentions of mine manifested when I didn't try anything and had, what's often called "divine indifference" towards it. There were other instances, where I followed the "steps" of manifestation and nothing happened.
The above observations and the fact that I've seen instant manifestation from very close quarters, has led me to this conclusion - for now
