$\newcommand\Legendre[2]{\genfrac(){}{}{#1}{#2}}$An odd prime $p$ defines the sequence $\Legendre1 p,\Legendre2 p,\dotsc,\Legendre{p-1}p$
of values of the Legendre symbol describing the quadratic nature 
of integers modulo $p$. We denote by $R(p)$ the longest run (largest subset of consecutive squares or of consecutive non-squares modulo $p$) of this sequence. Naïvely assuming the sequence to be similar
to a sequence of $p-1$ tosses of a fair coin, we should expect 
$R(p)$ to be of size $\log(p-1)/\log(2)$.
Since $\log(p)$ is the mean distance between $p$ and the following (or previous) prime, we should have
$$\frac{\log(2)}{n}\sum_{p\leq n}R(p)\longrightarrow 1$$
(where the sum is over all odd primes $\leq n$)
for $n\rightarrow \infty$.

This seems to be more or less the case numerically albeit
the quantity $\frac{\log(2)}{n}\sum_{p\leq n}R(p)$ is
strictly smaller than $1$ for the first few thousand initial values of $n$. This indicates a very slight bias for these sequences of Legendre symbols.

*Is this an artefact?*

(**Added:** A possible explanation is
the symmetry/antisymmetry of the Legendre symbol showing that we are sort of working with a random sequence of length $(p-1)/2$. This 'explanation' leads however to the opposite problem: The sum is too large, I believe.)

A few complements: The set of primes with a given value of the longest run should be finite. Computations for primes up to $50000$
suggest the cardinalities 
$$1,2,2,8,6,27,30,70,125,254$$
for the sets of odd primes giving rise to longest runs of length $1,2,\dotsc,10$.

First occurrences of 'records' have sometimes curious holes: The few smallest primes with maximal runs of length $29$ are smaller than
the smallest primes with maximal runs of length $25$, $26$, $27$ or $28$.