I am at the second point where you have the chance to be turned into one, before the soul cairn. If I decline, is this my last chance to be turned, or can serana turn me into one whenever I ask later?

I ask because I know if I accept it now, then I can cure it and be turned as many times as I want. But I'm wondering if I decline now, will I never be able to become one at all.

From what I understand Serana can turn you into a vampire whenever you ask her because well..she wants to spend eternity with you...and she gets your blood as a bonus.

I cured Harkon's disgusting gift and accepted hers..haven't cured hers to see if I can get it again though but everyone is saying its confirmed that she will turn you whenever asked.

I don't know if you'll lose the chance forever. This is what you should do save just before you accept, and then play for a while in Vampire Lord form, just walking around killing people. If you like it just keep playing if not load the previous save.