that all depends on how good you are at playing off situations, and improvising...if your routine ends up in the "and for my next trick", or "Now for this trick", you are probably headed in the right direction...that is the only downside to having a partner. I suggest that you and your friend get together and rehearse the show many times until both of you know exactly what is going on at any given time.
That is what we have been doing for practice. We get together and practice in front of a big mirror. What would you say is to long our show is between 30-45 min.