In today's world, what percent of marketing budget should be spent on digital marketing?

There is no correct answer to your question. You can spend one million dollars in one minute depending on the product/services you are selling and also the geographic region to cover. You decide on how much to spend on digital marketing based on evidence and not intuition. Normally whether you have millions, billions or trillions, good practice is always to start small and build up. You build up by optimization the campaign backed by conversion tracking and analytics. 

I;d agree with Stanley on this one. It totally depends on the prodcut/service and your brand. E.g. If your brand is Liverpool FC and most of your following is abroad and spread out, then by all means spend at least 70-80% on digital marketing. 

However if your brand is like Shan Masala Mix and your audience is more or less domestic or within the region and age dynamics is 30+ women, then put your advert in masala magazines and spend on tv adverts where the placement is between popular soaps. Putting 70% of your budget on digital marketing for a product like Shan Masala would not yield increase in sales and most of your audience (mothers and grandmothers) do not use instagram or twitter and facebook is only used to stay in touch with their families. :) 

Hope this helps!