About 6". I measured my bubbles (plus collector pose ponies) to get a rough idea of G1 MLP 'early' hair lengths, and my measurements were 2" forelocks, 4" manes and 6" tails. Which is luckily pretty easy to remember
And I personally prefer the 'original' style of manes and tails (especially on restorations), but you can always retail longer and then see how you feel about it. It's easier to cut the hair shorter than it is to make up an entirely new tail if you want it longer.