How much of your time in consulting is interviewing experts?

Time spent on interviews essentially depend on how many interviews you need to conduct in the case, which is in turn dependent on 2 things:

1. Data availability from secondary sources. If there is a lot of info from secondary sources, you could do with less interviews - i.e. just use interviews to check key hypotheses. Most of the time though, PE clients have a rough sense of how much info is available in the secondary domain and will seek to enlarge the scope (they don't pay DD fees just to get paraphrased market research material) - thus still requiring interviews.

2. Client direction on interviewee numbers: at the start of the project your team will typically agree with the client on roughly how many interviews you need to conduct. This is driven by a few factors such as (a) how many close competitors / meaningful players are really in your target's industry, (b) the scope of study (e.g. if you need to understand 2 functions like marketing capabilities and manufacturing processes within the industry, for each company you target to interview, you'd need to interview 2x as many people than you would if you were just focusing on marketing competitiveness), (c) any adjustments for lack of quality interviews, and (d) specific competitors your client is interested in learning more about

Overall, I'd say split between primary research (interviews) vs secondary research typically range from 25% - 75%. And yes - there are some cases where almost all data comes from primary interviews