To be honest, I don't think you can really 'beat' a Skate game.
I think it's all about what you want to do. Sure, they have a career mode and challenges but it's more about exploring the environments and finding some great hidden spots.
But anyway, to answer your question, if you play 1 & 2(Haven't got 3 yet so I can't comment on that) solidly and just do the career, depending on how quick you adapt to the controls and how quick you pick up on the timing you need, you should be able to beat 1 and 2 within the space of a week and a half each, if you put in practice and about 2 hours a day.
And Skate 1 was good but Skate 2 was better in my opinion.