Can you clarify how you would measure a percentage of a day (are you referring to a 24 hour period or the awake hours)? I think a more sensible metric would be to ask how many hours per day on average does a technical founder spend coding.
Averaged out in the first few months, it must be around 40-50% of my day. (~5-6 hours max).
I find it hard to believe that anybody could clock more than 70-80% sustained for more than a week. With 8 hours of sleep a day, you would get 16 hours of potentially productive time. 80% would equate to ~13 hours of coding leaving you with 3 hours for meals and general 'life admin'. That's not much.
The problem with raw hours is that it's less meaningful without the total hours. Ultimately I'm trying to understand how these people balance their days. Coding vs. non coding
During your start-up phase, on average, how many hours a day do you spend on:
a. Coding
b. Non-coding activities (related to your start-up)
That way you can deduce total hours spent (on average per day) on the start-up.
Also it would be useful for you to ask how long the 'start-up' period lasted.