How long until corneal swelling goes away ? due to allergic reaction to artificial tears

Artificial tears: Do not cause damage to your corneas. If your eyedrops contain other medications (e.g. Those that relieve eye redness) then you could have ocular surface problems. Regardless, you should stop the drops you are using and typically your symptoms should improve in a few days. If they persistn, then you should see your ophthalmologist to see if stronger medication is needed.

Several days: It is likely the preservative present that causes your corneal edema. Try the preservative-free single-use artificial tear instead and you will likely notice improvement in about 3 days.