The critical illness cover should be sufficient to cover for the treatment and loss in income during the treatment and recovery phase. Based on my research average cost of treatment TODAY is around ~20L. It is a function of many things but is a good starting point. However, this cost would continue to increase year on year due to medical inflation and of course the loss in come (during treatment and recovery) would depend on the income at the time of need. Depending on your current age and duration of the cover and your income, the cover could easily be upwards of 1 crore.
I hope above answers our question. At least partially. An easier way to estimate the cover is go through the journey at protectmewell.com - at the end of the 1 minute journey you get need analysis for term insurance, critical illness cover, retirement corpus and medical cover in an one pager report. The tool doesn't require you to share your email address or phone number and costs 29 (at the moment)
Discloser: I happen to be the co-founder of protectmewell.com