As a percentage? I don't know. Depends on the terrain and type of ride. Maybe 10%. More if I'm on my own, more if it's flat. Less if I'm with others or it's hilly.
Typically
On the flat, trying to move a bit more quickly than usual
Especially on the flat into the wind
To get a change of position, ease my wrists a bit
Also downhill while on the brakes, but that's not so much in the drops, as the loopy bit at the front of the drops.