So Squirt, my 1 year old Red Ear Slider, just got a new tank upgrade. I adopted him in February of '10, he was around 3 months old and the size of a quarter (around an inch). He's now around a year old and has grown like a weed. He's just under 3 inches. He started out when I got him, in a 10 gallon tank. A few weeks after, I upgraded him to a 29/30 gallon tank. Yesterday, he got upgraded to a 55 gallon tank, with a nice size canister filter.
He's loving the tank, though it's huge compared to him, but he's having a blast with all the swimming space. This tank should last him about a year, before he'll need to be upgraded again. At that point, he should be big enough to tell whether he's male or female, which will determine the next size tank I'll need. Around a 90 gallon if he's a boy, a 125-150 if he's a she.