I recently found out about Andrica’s conjecture: the square roots of consecutive primes are less than 1 apart.
In symbols, Andrica’s conjecture says that if pn and pn+1 are consecutive prime numbers, then
√pn+1 − √pn < 1.
This has been empirically verified for primes up to 2 × 1019.
If the conjecture is true, it puts an upper bound on how long you’d have to search to find the next prime:
pn+1 < 1 + 2√pn + pn,
which would be an improvement on the Bertrand-Chebyshev theorem that says
pn+1 < 2pn.