15%. But next year I'll be bumping it up since I can participate in "catch-up".
I don't have anything except 401k and HSA, being in MegaCorp.
The funny thing is I see many w*rk friends think 401k is enough, like somehow the government determined that $17000 is perfect for everyone and you can easily retire with it. Strange thinking, especially when MegaCorp has no DB plan, as most don't these days.
So, of course, I save another 15% or so in taxable accounts of various types. I'm at that period in life and near ER that I can do that. I know everyone can't. At the end of the year, any extra also gets saved, so the % ultimately goes up every year depending on the year's expenses.
Oh yeah, I also participate to the max in an HSA, but that's not much you can defer, although every bit counts.
I tell the fresh kids out of college who totally eschew the 401k to start with *something*, what ever they can. If they are at zero (most are), I encourage they start with 2% at least, better if they can reach the company max. Then put in a plan to increase every year. That's what I did, and I'm glad for it. Our plan has an "auto increase" option which I encourage for the youngsters.
I think the 10-15% comes from the average of knowing so many are at 0%, they keep the goal realistic. In my opinion, you have to strive for more. I didn't go beyond 15% until my 30s, although effectively I may have reached it earlier if I include the extra principal I threw into the mortgage back then.