Dunno what beans you're using but 15 seconds sounds like a longish time for a double on a SJ-E. Generally I'm in the 10-12 second range. To calibrate the timer, my strategy was to time with a stopwatch how long it took to grind a given dose (eg. 18g), weighed using a scale, when using the 'manual' button, and then adjust the double dose button to this time. Doesn't really make sense to start out using time as guide without calibrating for mass, or at least grind quantity in the basket.
Just a few other general tips with this grinder:
1. Bear in mind a fair amount of grinds will be kept up in the burrs and anti-static mesh. I usually purge out a double shot at the start of a coffee-making session to clear out stale grinds, and also purge when making a grind adjustment.
2. Make sure the grinder is running when adjusting the grind. This will avoid the burrs getting jammed up and save you an unneeded disassembly.
3. The timer will need to be adjusted with different bean types and other variables (eg. a coarser grind setting generally = shorter grinding time and vice versa).