I managed to find the answer in pci express card electromechanical specification revision 2.0
Table 4-2: Add-in Card Power Dissipation
Not quoted, but interpreted :
Initial power up max
X1 = 10watt, X4/8 = 25watt, x16 = 25 watt.
If add in card configured as high power device:
X1 = 25watt, X4/8 = 25watt, x16 = 75watt.
These limits appear to be the same in the newer revisions 3.0 and 4.0. I haven’t look at 5.0 though.