How long does second FUE treatment start to grow?

No, second FUE grafts do not take longer to grow then first .Regrowing hair is thin, undersized and not bright untill 4-5 th months of hair transplant. İt's normal for new growing hair starting to get native characteristics, waviness, curl in first 4-5 months..This actually takes 10-13 months for hair to become full mature , as thick and bright  as native hair. 

I wouldn't be worried yet, a second transplant may not necessarily grow at the same rate if it's a different sized procedure, in a different area, done by a different doctor, etc. so just treat each one separately. If you don't start seeing new growth starting within the next 2-3 months, that's when you should begin to feel concerned.