The paradigm shift that vim asks here is to think of normal mode (it is not "delete" mode, it is "normal" mode) as well normal, the place to be. This shift in thinking can take some work to get used to, but in vim thinking, it is the insert mode that should be used more rarely and that it is insertion and insert mode that requires the `ESC` to say "I'm done inserting". (And thus delete line is only ever `dd`; `ESC` is not a part of deletion, it is a part of insertion.)
One of the things that typically helps to get into this mode of thinking is to take advantage of/get used to using the home row "arrow keys" (hjkl) for navigation. Instead of moving your hand to the arrow keys (or worse, all the way over to your mouse), don't leave the home row and try navigating in normal mode with hjkl.
It certainly depends on what I'm working on, but as the saying goes code is typically read a lot more than it is written and there are definitely days of bug finding and editing where you hit or dip below 10% in insert mode, spending much more of your time in other modes or navigation. I'm not sure my average day is 10%, the mean is probably more like somewhere between 25-40% when I was most heavily using Vim to write code.
I'm more often going to be in insert mode writing prose, but that is a different flow state. Also, even with prose it sometimes feels like you spend more time editing it than writing it.
One of the things that typically helps to get into this mode of thinking is to take advantage of/get used to using the home row "arrow keys" (hjkl) for navigation. Instead of moving your hand to the arrow keys (or worse, all the way over to your mouse), don't leave the home row and try navigating in normal mode with hjkl.