Can we reschedule FP time?

Hi, all,
I did FP in March 2005 (485 still pending), and I am planning to visit China this summer for about a month, these are my concerns:
1)In general, when will the second FP be since the first FP? 18 months?
2)if I get the second FP notice but I am still in China, can I let my friend call USCIS to reschedule another time? or should I call by myself?

people said,FP time is fixed recently, not flexible any more, any other suggestions? really appreciate.