I agree with Sebastian and Miguel about "por pagar". About your second question, I can tell you that the information given with "¿Cuánto tiempo has estudiado?" is like saying how much time you have studied in your life or until the moment in which is made the question, but notice that both are situated in the past and you're not giving information about if the person is studying now. Instead, when you ask "¿Cuánto tiempo llevas estudiando?" you're making the question to a person that you know that is studying now. I think it's more like "¿How long have you been studying?" In the first two minutes of this video you can see an example of this use: https://youtu.be/pMDqXr9lc6E el vídeo se llama: "Spanish grammar: I've been living here for 5 years"

I agree with Sebastian. You can say "Tengo muchas facturas por pagar". And for the second question, "llevar" is more likely to be used in everyday language. Another example: "Llevo todo el día esperando" sounds more "everyday" than "He estado todo el día esperando".

Hi, Rebecca: 1. Para pagar doesn't sound that good to me. You could use "por pagar" (in fact, I prefer this one to "que pagar). (it sounds more natural if you drop the pronoun "yo") 2. I'm not good at grammar, but it seems to me that both verb phrases carry the same meaning, that is, they both refer to an ongoing situation.

You can use the alternatives forms in both cases. But keep in mind that in sentence 1 you shouldn't use the explicit subject before the verb