Hi Guilherme,
I've been doing this for the last year. And to be honest, if you make small batches of 250 ml (30 Paracetamol tablets 1000mg, 20gr NaOH and 50gr Sodium Sulphite) you'll most probably use it before it get bad. And when (or if) it gets bad, being so easy and cheap.. just make another batch.
In my case, I had a batch that lasted at least 6 months, without any "special" protection (the last solution was on an almost empty bottle, and worked the same.. to me).
It's fun.. enjoy!