  We revisit the concept of turnaround radius in cosmology, in the context of
modified gravity. While preliminary analyses were limited to
scalar-tensor/$F(R)$ gravity, we extend the definition and the study of this
quantity to a much broader class of theories including also quantum $R^2$
gravity. The turnaround radius is computed in terms of the parameters of the
theory and it is shown that a deviation not larger than 10\% of this quantity
from its value in Einstein's theory could constrain the model parameters and
even rule out some current theories.
