Purchased points usually show up immediately (you may need to log out of your account and back in), but officially they say "points will post within 72 hours."
Be aware that buying points is not a good deal. At 3 cents per point (or 2 cents per point with the current 25% off offer) you are essentially paying more than the actual value of the points (~1.4 to 1.9 cents per point). Points purchase is usually best for "topping off" when you just need a few more points in order to purchase a specific flight.
