It can take a little while for it to acclimate to your system before it starts to grow, and I would slowly dial your lights up from 1/4.
Once it acclimates & starts spreading, the growth rate of GSP is pretty quick, but you need to keep perspective in mind..... Everybody's tank is different, but if I were to put a number to it, I'd guess that GSP may expand roughly 20% to 25% monthly. If you've got a little frag that's an inch across, 20% of that is going to be disappointing if you're expecting explosive growth. But, once it reaches 5" or 6" across, 20% of that is going to be significantly more growth in the same period of time.
And, be careful what you wish for....... once GSP does really start to take off, if left unchecked, it can (and will) overrun & suffocate other corals. My best advice is to have it on it's own piece of rock..... once it gets close to leaving that rock, trim it back.
Wanna calculate the cost of that new reef tank?? Take your worst nightmare, double it, and you're halfway there.