How long does it take to kill aphids?

Hose them down.

Run water all over the plant, making sure to target the underside of each leaf. Repeat this process every few days until you've successfully eliminated all aphids, which could take up to two weeks.

How long does it take neem oil to kill aphids?

The advantage of using foliar sprays is their ability in the control of aphids used as spot treatments on indoor plants, as well as the fast dissipation rate. A neem spray will generally evaporate with no residue in around 45 minutes to an hour, rendering plant surfaces safe for beneficial insects or pet contact.