There is a very relevant article A Review of Issues of Dietary Protein
Intake in Humans International Journal of Sport Nutrition and Exercise Metabolism, 2006, vol. 16, pages 129-152.
The rate at which the intestines absorb amino acids depends upon the protein source, and, according to table 2, this varies from a low of 1.3 grams per hour for egg protein to 8-10 grams per hour for whey isolate.
Rudman et al. (27) found that the maximal rate of urea excretion (MRUE)
in healthy individuals was 55 mg urea N ∙ h-1 ∙ kg-0.75, which is reached at an intake
level of 0.53 g protein N/kg-0.75 At higher protein intakes there is no further increase
in urea excretion rate, but a prolongation of the duration of MRUE, often in excess
of 24 h
(the "-0.75" above is an exponent, as in kg^(-0.75))
So absorption by the intestines is not necessarily the only limit to consider.
The dangers of excessive protein intake should not be underestimated and
have been recognized historically through the excess consumption of lean wild
meat by early American explorers leading to a condition referred to as “rabbit
starvation syndrome,” in which symptoms included nausea and diarrhea followed
by death within 2 to 3 wk
The review also states:
This “slow” and “fast” protein concept provides some clearer evidence that
although human physiology may allow for rapid and increased absorption rate of
amino acids, as in the case of WP (8 to 10 g/h), this fast absorption is not strongly
correlated with a “maximal protein balance,” as incorrectly interpreted by fitness
enthusiasts, athletes, and bodybuilders. Using the findings of amino acid absorption
rates shown in Table 2 (using leucine balance as a measurable endpoint for protein
balance), a maximal amino acid intake measured by the inhibition of proteolysis
and increase in postprandial protein gain, may only be ~ 6 to 7 g/h (as described
by RPT-WP, and casein) (38), which corresponds to a maximal protein intake of
144 to 168 g/d.
So yes there is a maximum rate somewhere in the 144-240 grams per day range, but it is not necessarily safe to approach the maximum.