I couldn't find any reference to minimum lengths of pipe in the 2002 Edition of B31.3.
However, I did find this interpretation 14-03 Dated July 22, 1995, which states there is no mimimun requirements for the minimum length of pipe sections between welds.
With reference to ASME Section IX, QW-302.2 When the welder or welding operator is qualified by radiographic examination, as permitted QW-304 for welders, and QW-305 for welding operators, the minimum length of coupon(s) to be examined shall be 6 in. and shall include the entire weld circumference for pipe(s), except that for small diameter pepe, multiple coupons may be required, but the number need not exceed four consecytively made test coupons.