Most moms begin a bottle because they want the freedom of allowing someone else to feed or out of necessity because they are going back to work. If you choose to nap and let Dad feed make sure to fully empty the breasts by nursing first. If a bottle has been given while you were asleep or running errands, pump when you return even if you have nursed immediately when you came through the door. Anytime a bottle was given, baby will not be as hungry and therefore not empty the breast as well and residual milk can cause clogs especially in the early weeks of breastfeeding.
If you are not going back to work or want someone else to feed, a bottle is not an absolute. You may wait longer if you wish, but some moms who wait longer than 8 weeks find it difficult to coax their baby into taking a bottle. So if you are going back to work, the best window may be 4-8 weeks. If you are not going back to work, but find yourself in an emergency situation that you have to be away from your baby, the baby will normally take a bottle from another person if you are gone very long. If you are not gone more than 3-4 hours, the baby may also wait for you. In any case, it is important to have stored up at least 24-48 hours worth of breastmilk in the freezer. Here is a breast milk intake chart to help you determine how much you need based weight.