I've done it before, the longest it's taken me is about 6 hours. Two ways tondo it, which is story hard missions get someone high level torun with you. (You'll be under leveled at some point, so you'll need to do something else, like crucible to level a little.) usually level 19-20 at the end of Mars missions (doing all strikes and bounties available) Or, you can do crucible, all bounties possible and earth and moon missions (hard) and the two strikes. < this was fastest for me. But depends how good you are online.

Thanks man, i like the second way better