Some are already available online deposited at K, P, G, E, BM, KFRI, S, etc.

You can also visit Database of Indian Plants at

https://sites.google.com/site/efloraofindia/home

There seems to be no certain time frame for this. Botanical Survey of India people (who have maximum number of herbarium specimens) are scanning their herbarium specimens to make them available on the net. During the last few years they have made available the published volumes of Flora of India (http://efloraindia.nic.in/efloraindia/homePage.action) and few other literature. I hope they will come up with Digital Herbarium within few years. For the images of majority of Indian plants plants efloraofindia (@ https://sites.google.com/site/efloraofindia/home) is a good resource.

you will find link to online digital indian herbarium repository here https://herbnews.net/how-to-prepare-herbarium/