Informal vs. formal acceptances

[akraticfanatic]

So I've been accepted to four programs. Of these, I've received formal acceptance letters from two and a financial aid letter from one. Of the two from which I've only received informal emails, I received them 2.5 and 3 weeks ago respectively. I tried emailing both schools and got an unhelpful response from one and no response from the other. For people who have been through the app process, is this normal? How long does it usually take?