From the time of primordial nucleosynthesis (less than a second). This nucleosynthesis also produces, by the way, correct ratio of H/ D/ He which we see today.

We may also say that actually physics is the same as now even way closer than a second to the beginning of Big Bang by observing behaviour of ultrarelativistic matter in high energy accelerators and especially in high energy cosmic rays (which sometimes have energies about 50-100 Joule per particle, which corresponds to temperatures about 10^22 K or about 10^-20 sec after Big Bang, and they all behave EXACTLY same way as low energy particles. This is direct indication that h, c and e are still SAME at least at that high energies in very early universe.

By the way, it is only 7 orders from Plank energy (which is about 10^9 Joules/particle).

So, it is believed that h, c, e, and G are likely "hold" till Plank energy level, although direct experimental evidence that physics is "same" is up to energies about 100 Joules.