OohFeeshy
It's only forever; not long at all...
When mixing the patircularly fancy tailtypes (ie, ct, dt, pk [not fancy, i know...], with sd, d, vt), you won't get anything worth much until at the f2. Since most of the desirable traits are reccesive, the f1 will generally have tails akin to the mutts you'd find at your local lfs. So unless you have, as Synirr said, a particular aim, eg, a CTPK line, it's not worth it. But breeding two 'plain' tail types together, (by that, I mean HM, SD, D, PK) together, the fry should be fine, you just have to keep an eye of the tail spread of the parents so you don't end up with a load of VTs.