A week or two weeks or a month or more. Decide on a time limit, put the thing away, and for the whole time don't look at it or work on it. Do something else.
Then come back to it. If it sucks as bad as you think it did when you put it away, throw it out. If it seems to need work, put it away for a shorter time and then make changes. If you love it and you have fresh inspiration, get busy on it.