What is the maximum run length in PCIe gen 3 serial stream?
PCIe gen 3 uses 128b/130b line coding, so absolute worst case run length would be 129 bits (128 scrambled bits + half of the 2 bit sync header). This assumes that the scrambling PRBS and transmit data exactly coincide and cancel to zeros for the entire 128 bit data block, which is possible but extremely unlikely.  Note that in contrast with 64b/66b, the 128b/130b line code does not use a self-synchronizing scrambler.  The DC wander is statistically bounded (as opposed to absolutely bounded) due to the scrambling.