The max length will vary on the specific tape you've used and the effect may also be more or less noticeable depending on the tape. Some tape as on-board current-control IC to allow for longer lengths even.
To half the voltage drop (which would be ideal) you'd need to connect all 5 cores to both ends. Connecting just the "+" at both ends won't be as good, but would probably mitigate things enough to not be noticeable, especially if only 6m.
There are actually 4 "-" cores, not just 1, one for each colour.