I've been playing the Piano for 7 years now and I would really like to learn the Cello, how long do you think it would take me to learn it, and is it hard to learn.

I don't play the cello, but I think it won't be too hard to learn if you work really hard. I mean, I've been playing the flute for 11 years and the piano for a few months and this autumn I'm going to start playing the violin, though I've heard it's really hard to learn.
And... I think every instrument has it's own difficulties. So... If you really want to, you could try, right?
I can't know how long it will take you to learn an instrument... It's so many things that has to be with that. Like, how much will you practise, and do you have easy to learn and other things like that.
But, as I said, I don't play the cello, I'm only a person who's going to start out with a third instrument this autumn.
Hopefully that helps a litlle ^_^