I think you should keep thinking about your intention until you really KNOW it is going to manifest. However it's when you really know it's manifesting that you're actually really intending it. So it's a bit paradoxal that you can detach from it when you really intend it.
Let's compare it in this way it may be clearer :
Every morning you know that the sun is going to rise. You know it and you're so sure of it that you don't even need to think about it. It's completely part of your reality.
When you know with the same certainty that your specific intention is going to manifest, then you can stop thinking about it.
As Acting Like Godot says, it's important to not put negative thought into your intention, because it's like thinking "I'm not sure this is going to manifest".
If you know, deep down, that something is going to happen you don't need to give it much thought. However if you can't really feel it's going to happen then use positive thinking (and acting) until you do.