Actually, when I think about it, there are more dialects that do than don't, so all over, I suppose. There are some rural dialects that might do it differently.
I think it happens when the word is unstressed, so it might be something like what happens to some function words in English (the word <a> is usually a schwa, for instance, because it's unstressed, but it is pronounced /eɪ/ when stressed).
Oh, and you would also do it if you use it in a contraction it with <ikke>, that is <Ska'kke> which would translate to <am not going to>. This is not officially a thing in Norwegian, but it happens. Like saying <wanna> or <gonna> instead of <want to> and <going to> in English; it's done in speech, but frowned upon in writing.
All the modal verbs (and a lot of monosyllabic regular verbs) can contract with <ikke> in this manner and most lose their final consonants if they have any. There's actually a song by a Norwegian celebrity that uses this to great effect. Here's a link: https://play.spotify.com/track/2DzCc1SryTgj7c4QH41aKc