Okay, first I'd require a good amount of time at least a few weeks. Second, the person would need the appropriate training equipment. For combat feats training dummies would be good, but actuall living opponents would be better. Meta magic feats would be alone with the spell book trying to figure out how the feat will interact with the spells.
Now, if these feats are going to be learned as one of the normal feats a character gets then not much outside of this should be needed. Perhpas a bit of gold spent for the training area, or special materials. I wouldn't want to punish a player to much for trying to develope something cool and new into his character (this is of course assuming that you, the DM, approve the new feat or ability).
Now, if you wnat to add the possibility of failure, I'd require a roll of spellcraft, knowledge fighting (with a bonus equal to BAB so fighters actually have a chance with out dumping skill points into something that will never be used), or something to all this training.