The norm is that you will always have time to attack back after a block.
But there is a stun mechanic, and it most often comes into play if you are blocking a much heavier weapon than your own, that is held as well.
So if someone attacks you with a heavy ass weapon, holding it as well, you can expect a stun, and might have to block the next enemy swing too. This is something I have to be aware of with my light katana. Someone with a held voulge or GS attacking me, I know I very likely may have to block twice. 1 handers are also often light, so they suffer from the same disadvantage.
If the guy attacking you is not holding/charging the attack, it most likely won't cause a severe stun though.
(I guess you know, that you do more damage if you hold the attack for about 0.8 secs. )