TPM 3VSB power capabilities
Every motherboard design is different. The designers choose various parameters to optimize the design for various factors including:


BOM cost of motherboard.
Reliability of components.
Allowable temperature rise of components.
Voltage regulator output capability design margins.


For sure the motherboard includes a voltage regulator for converting the 5VSB to 3V3SB. The design margin for that regulator could vary over a fairly large range from 50% to 90% or more. (Design margin being the load current requirement divided by the regulator's designed current supply capability). Because of these varied factors you could not generalize the available current from a TPM connector by any other means than evaluating a TPM module by itself.

You could try to determine by inspection whether the voltage regulator used on a particular board is a buck mode switcher or a low dropout linear regulator. Due to the increased efficiency and lower temperature operation of a modern switcher regulator it is more likely that the switcher would have more design margin than a linear regulator.