From the day I received the informal offer (the letter came 2 days after), I was given about 10 days to answer (it was the second half of March and they asked for an answer before April 1st).
But if I had to guess, for someone receiving an offer later like this week, I would expect that they give less time due to practical reasons. For example, at my host University* you can only apply to the Visa until June 1st and there is also the housing allocation, etc, so probably they cannot wait much longer for a student to decide. Of course this may be different if you already have all these stuff sorted out. Anyway, this is just me guessing - you probably can ask Judith for an extension if you need and she will tell if it is possible or not.
Feel free to drop a pm if you have more questions.
* students at TI needs to elect one of the three mantaining universities (VU, UvA or Erasmus) as their host. It changes basically where you will be able to get housing (Amsterdam or Rotterdam).