Special abilities (SA) require 50 skill points... if you have a fast trainer, then he can maybe get the SA within a season. I'd only recommend training a SA if your player is young, definitely under 21, otherwise you might be wasting 2+ seasons to get that ability when it's quicker to just buy a player from the auctions.

You could always experiment with a youth signing. Choose one that's reasonably well rated, then aggressively train an SA. Make sure he plays most games, as that gets you trained up quicker. I feel the least risky kind of players to train who won't "break" a team by playing are ST and the AMC/L/R. You certainly wouldn't want to play an 18-year old GK who's training an SA in every single game, but he'd probably be okay in games that don't matter (e.g. second leg of a cup game where you're already 7-0 on aggregate).