It’s best not to give an answer with specific time frames, for several reasons. First, children have a rather limited comprehension of time, and won’t be comforted no matter what you tell them. To a preschooler, a week might as well be a month which might as well be a year which might as well be half their life. Second, there is absolutely no way to predict the process of grieving or how long it will take.
Instead, talk in steps. Say something like: “I don’t know how long it will take. Losing someone you love so much is very hard, and nobody gets over it right away. But I do know one thing: all injuries get better with time. So when you wake up tomorrow, you’ll be just a tiny bit better than you were today. And the day after that, you’ll be just a little bit better than tomorrow. You may not notice an improvement from day to day, and there may be times when things feel worse and it seems like you’re going backwards. But as the days pass, each day the hurt will go away little by little, until we don’t feel it every day like we do now.
Also explain that although you’ll never forget the person or stop loving and missing them, eventually it will get to the point where it won’t hurt to miss them. You’ll talk about all the good times you had together and you’ll wish they could be here to share your life now, but the hole in your heart will be filled with the love of other important people. Though your lost loved one can never be replaced, we can draw love from many different people, and eventually that love will heal the wound that was opened up by death.