Since in my long Carth campaign, I only built one academy in Carthage as soon as I could, it lasted for at least 50 years (for the rest of the game) filling my faction members with retinues.

RedKnight - to dump usless retinue, you could bribe a no-star faction member off someone, fill him up with unless retinue and use him for ferry missions. I like to have a guy that visits key mercenary locations to raise mercs for me.