I often here people say something that contradicts the rules I was taught back at school:
It's been long since I've seen my parents.
I thought I should say: It's been long since I last saw my parents. or It is ages since I saw my parents.
But defenitely not present perfect after 'since'.
So why is that so?
The sentence you quote, 'It's been long since I've seen my parents' is not one you'd hear many native speakers say. It's not grammatically correct - the form, as you rightly say should be either 'It's been a long time since I last saw my parents', or 'I haven't seen my parents for ages.'
But, there again, native speakers of English do sometimes make grammatical mistakes!