It absolutely depends upon the quality and quantity of your probiotic. You want to take one that is in the billions count and is a good brand. You will then want to double dosage at the beginning to saturate your system.
Are you looking to help replace and replenish good bacteria from killing bad bacteria with antibiotics? Probiotics will not kill bad bacteria!
I agree about the quality/dosage of your probiotic. I think it also depends on the particular imbalance you have. If you've developed something limited from a short-term medication, for example, I would think that would be faster to clear up than something more long-standing. Are you willing to include dietary changes in your overall plan, too? That can make a huge difference!