Board Thread:Spoilers!/@comment-27060727-20170222030619/@comment-25375217-20170222205701

Once episodes are roughly 43 minutes long and there are 7 songs. 43 / 7 is 6.14 (rounded), or 6 minutes and 8 seconds each. Taking into account maybe two minutes for dialog between each song, that's 3 minutes and 51 seconds each.

Obviously, they won't all be the exact same length, so I'd say roughly 2.5 to 4 minutes per song depending on what's going on. Emma's solo will likely be on the shorter end of that (since there's only one character to handle), while Regina's sing-off with the Charmings will likely be on the longer end.