Pre-existing disease coverage waiting period - diabetes

I was diagnosed with diabetes about 18 months ago. I am planning to buy a health insurance policy now. I understand there is a waiting period for pre-existing diseases. Can someone explain how long I need to wait before I can claim for treatment related to my diabetes? Also, will the waiting period be different for different insurance companies? I have been comparing policies from Niva Bupa and Bajaj Allianz, but I am confused about their waiting period clauses. Any guidance would be really helpful.