As with most nutrition questions, the answer is complex when you start to dig deep.
The simple answer is that in a single dose about 0.5g per kg body weight is probably a sensible upper limit. Long term, there is no evidence of kidney or other organ damage, or effect on mortality with high protein intakes. That’s doesn’t mean to say an excessively high protein diet isn’t bad for you – rather no studies have been conducted to prove it is (or isn’t). As ever, strive for a balanced diet, with sensible use of supplements if required, and you’re not going to be pushing at the boundaries of what’s good/bad for you.
The more complex answer is you need to consider protein over-consumption on two levels.
1. The acute effect, i.e. short term or single dose. Here what can happen with over-consumption is that the protein ends up being oxidised, metabolically this means it is turned to glucose and/or fatty acids and contributes to your body’s fuel. Hence excess protein (contributing to excess fuel overall) is still potentially going to make you, or keep you, fatter if you’re in a calorie surplus. The deamination of the amino acids as part of this protein catabolism also means dealing with the amino groups (ammonium ions) NH4 which may lead to point 2. An excess amount of protein is likely to be around >0.5g/kg in one go, if in a meal, but if taking supplements it is likely lower and more like anything over 0.3g/kg will be oxidised.
2. The chronic, or long term effect. Here the effect of excess protein is a demand on processing the nitrogen (NH4 ammonium) produced through protein deamination. This will need to be excreted via the kidneys via urea production. In addition, there may be alterations in acid-bases balance as a byproduct, which may impact bone resorption, in order to buffer this shift in homeostasis (i.e by liberating calcium from the bones). However, the evidence for this is not entirely clear. Protein consumption may actually be slightly protective to bones1. There is no strong, or consistent evidence on kidney damage or morbidity/mortality with high protein intakes. But then again, it is difficult to longitudinally prove, as no one has performed longitudinal studies (over several years) where you can satisfactorily attribute effects directly to protein alone. Nor are there any studies looking at long term (years) protein supplementation.
1 ‘Dietary protein and bone health: a systematic review and meta-analysis’ Darling et al, 2009 https://www.ncbi.nlm.nih .gov/pubmed/19889822