What is a reasonable percentage salary increase for tenure and promotion from assistant to associate professor in the US?
Data on faculty salaries is available from the AAUP survey:
Associate professors make, very roughly speaking, 10% more than assistant professors. However, since professors get a few percent raise a year, you can estimate that the difference between the most experienced assistant professor and the least experienced associate professor is about the typical annual raise. So, I think the answer is "not much." However, it varies widely by institution, school, department, governance model, etc.