Depends on Seniority
I'd say the percentage of time spent literally typing lines of code is inversely proportional to your seniority. Junior engineers get to spend a larger percentage of time writing code because they are not trusted to design/architect it, meet with stakeholders to gather requirements, fight with project/program managers over deadlines and deliverables, give presentations, etc. One answer said "code reviews should take less than an hour". Well, that depends a lot on how many other people on your team are doing them, how quickly they are writing code, and how seriously you take the responsibility yourself.
Depends on Cycle
At the beginning of a new project, there will be more time spent on investigating tools and technologies to be used in the project (libraries, frameworks, 3rd party tools, etc.). At the end there will be more time spent on fixing bugs, deployment, documentation, etc. The middle is likely where most of the coding time is spent. Or, if you're stuck on a maintenance team/project, you might be fixing bugs 100% of the time for weeks or months on end.
Depends on Project
The highest density of code writing probably occurs when making a prototype/proof-of-concept. There, any pretense at maintainability is tossed out the window and you are racing to assemble something held together by duct tape and crazy glue. Then I could easily see someone spend 8+ hours a day writing code, for multiple days in a row. Outside of that, the only kinds of projects where I can see sustained code-writing are things like framework migrations, large refactorings, or other kinds of upgrades which are mostly mechanical and involve a lot of busywork.
On the other end, I would expect to see the least amount of code writing when developing a brand new technology that will require new tools/libraries/frameworks that you/your team have not used yet. Evaluating the alternatives to determine best fit, negotiating requirements/deliverables, and all the other administrative overhead means that you may spend weeks without writing any code at all.
Depends on Scale
If you are working in a startup, I'd expect your proportion of code-writing to be fairly high, especially because of the premium on delivering code vs. maintainability, etc. Running a startup is a lot more like building a prototype (and may literally be that) than maintaining a legacy beast. If you are working in a Fortune 500 company, you could be in almost any kind of team working on almost anything. But the odds are much higher that you are stuck with a legacy codebase and a bunch of technical debt that you are trying to bail out of. You probably spend way more time going to meetings and coordinating with other teams than the startup folks. You probably spend more time shooing away the project and program managers than reviewing code. You probably have to spend a lot more time thinking about the wider impacts of your technology decisions because your service/app might have 100x the usage of a smaller company, which forces you to investigate more tools/services/frameworks than you would in a smaller company. All of these factors reduce the time spent coding.
If you're a junior engineer in a startup/non-technical company, I would be shocked to hear you only writing code 15% of the time. If you're a senior engineer in a large company with a significant tech investment, then 15% coding time sounds about right.