Yea i find mine growing about 1.5 inches a month roughly maybe even 2 inches some months... Its all about how you feed it and amount of water changes you do. I do I 30% water change every other day.Sounds like a runt or stunted perhaps, at 5" any aro should be growing around an inch a month if housed and fed properly. If your jardini hasn't outgrown a 4' tank in 3-4 years there's something wrong, quite possibly the tank size.
IMO you should be upgrading to a larger tank (6x2x2' at the very least) by the time it gets to 12-14", so if you base it at around an inch a month you could have around 10 months.