Funnily enough, this question came up in my game on Sunday as well.  Finding nothing to indicate otherwise, I allowed my player to charge as often as he wanted, provided he had at least 20' between him and his opponent and a relatively straight path.

Agricola wrote:

I thought I saw something somewhere about this. Don't have books at the moment-maybe the Cataphract class description, I know I remember reading something about charging-like only able to do every third round, needing two rounds after the charge to "build up a head of steam" (my words) or something along those lines. 

Spot on: it's in the cataphract description, but it refers only to mounted charges:

"On an open field, two rounds are needed to prepare subsequent mounted charge attacks; thus, a mounted charge attack may be effected every third round."

It's repeated in the Combat section on the mounted charge, but no mention is made for regular, horse-free charge attacks.

Looks like you fellows have puzzled out my intent with the charge rules. If there is anything else I can answer to clear it up, just let me know.