mmm, I've known this number, but it's a long time ago I had to use this. I tend to remember it is more than 6-8 seconds, like 12-15 seconds from ground idle to max power. I've had several occasions where we had pilots reporting slow acceleration, most of the time due too a bad MEC (main engine control). This controls the variable bleed valves and variable stator vanes. Maybe the high stage bleed valve is sticky/worn, so after 70% all is going well.