How long can an insurer exclude coverage for a pre-existing condition on a group health insurance policy?

Yes, group health insurance ( group mediclaim) plans can cover pre-existing diseases and their treatments from Day 1 of coverage under the policy. It would be best to take the group health insurance policy with a waiver of waiting for pre-existing illnesses. Removal of pre-existing disease clause needs payment of an additional amount, called loading, to get this benefit under the policy.
Coverage of pre-existing diseases is one of the most significant advantages of group health insurance policies. 
There are three types of waiting periods in any health insurance cover – 

1. 30 Days waiting period – this says that the first time you take the health insurance policy, any claim, other than accidental claims, will not be paid in the first 30 days. This clause is not applicable from the second year onwards.
2. 1st Year / 2nd Year Waiting Period for specific and named treatments like cataract, hernia, hysterectomy, fistula, piles, and stones
3. The pre-existing disease waiting period is applicable mainly for four years, but some policies also have it for fewer years.

The advantage of group health insurance policies is that waiting periods applicable to you in a retail policy can be waived off. The waiver of this waiting period happens by paying an extra premium called ‘loading’ to the insurance company.
Once you have taken waiver of waiting periods in your group health insurance policy, you can claim from the date of coverage for any hospitalization arising from pre-existing diseases/conditions.