It depends a bit on what you're brewing.
In a light beer 5% will give the subtlest hint but will be virtually undetectable in something like a porter.
10% is probably the subtle taste zone for a porter.
I wouldn't go past 20-25% myself, depending on style. I've tasted a full-on Rauchbier and it tasted like someone had brewed it with a leg of ham in the fermenter.
The other consideration is which smoked malt. Weyermann and Hoepfner are the two generally available here and one of them is noticeably stronger flavoured than the other, but I can't remember which.
Can anyone help my beer-addled memory? :blink: